He gave him advice and he did not pay any heed; should he cut off ties with him?
Question

                I am a young religiously-committed man, and I have a brother in faith whom I love for the sake of Allaah. I saw him at the traffic lights with music playing loudly in his car, and I advised him in a direct manner without letting him know that I had seen him. He showed that he was sorry and had repented, and – according to what he said – he removed the music from his car and computer, praise be to Allaah. But a few days later, I saw him participating in chat rooms, talking in an indecent manner and writing down the names of songs. I recognized him by his screen name and his avatar. It seems that sometimes he talks on the internet with some of his female relatives who are not his mahrams. 

Should I cut off ties with him, or should I take him as he appears to be, knowing that he accepts the advice without acting on it, and that outwardly he appears to be committed? His friends know him as such, but I know something about him that they do not know.
Praise be to Allah.The one who 
    disobeys Allaah either disobeys Him openly or he conceals it. The one who 
    disobeys openly should be forsaken – even if it goes on for a long time – if 
    forsaking him will deter him from that or will make him do it less. 
If he 
    conceals it but it so happens that you see it, then he should be rebuked in 
    secret and he should be advised but his fault should be concealed.  
The 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) said: 
    “Whoever conceals (the fault of) a Muslim, Allaah will conceal him on the 
    Day of Resurrection.” Agreed upon.  
Muhammad ibn 
    al-Munkadir saw a man standing with a woman and speaking with her. He said: 
    “Allaah can see you, may Allaah conceal us and you.” 
In such 
    cases it is prescribed for you to forsake him on your own, if that forsaking 
    will have an effect on him and will not result in any greater evil, such as 
    his committing sin openly after having concealed it, or doing more of it 
    after having done only a little, or doing something that is worse that that 
    for which you forsook him. 
Shaykh 
    al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: 
Whoever 
    commits evil openly must be rebuked and forsaken and criticized for that. 
    This is what is meant by the phrase, “Whoever becomes shameless, talking 
    about him is not gheebah (backbiting).” This is unlike one who conceals his 
    sin and seeks to hide it; he should be concealed but he should be advised in 
    secret and forsaken by one who knows his situation, until he repents.” 
Majmoo’ 
    al-Fataawa (28/220) 
Al-Bukhaari 
    (may Allaah have mercy on him) included the hadeeth about Ka’b ibn Maalik 
    staying behind from the campaign to Tabook in a chapter which he entitled: 
    “Chapter: What is permitted of forsaking one who has sinned.” 
Al-Haafiz 
    Ibn Hajar (may Allaah have mercy on him) said:   
What he 
    meant by saying “Chapter: What is permitted of forsaking one who has sinned” 
    is to explain the kind of forsaking (or boycotting) that is permitted, 
    because of the general meaning of the prohibition on forsaking a person for 
    no legitimate reason. Here he explains the reasons that justify forsaking 
    someone, which is when a person has committed a sin, and it is justified for 
    the one who found out about it to forsake him so that he will refrain from 
    that. 
Fath 
    al-Baari (10/497) 
It was 
    narrated from Sa’eed ibn al-Jubayr that a relative of ‘Abd-Allaah ibn 
    Mughaffal threw small pebbles and he told him not to do that. He said: The 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) forbade 
    the throwing of small pebbles and said: “It does not kill the game or kill 
    (or hurt) the enemy, rather it breaks a tooth or puts out an eye.” Then he 
    did it again and he said: I told you that the Messenger of Allaah  
    (peace and blessings of Allaah be upon him) forbade it, then you throw small 
    pebbles again. I will never speak to you. Narrated by al-Bukhaari (5162)and 
    Muslim (1954). 
Al-Nawawi 
    (may Allaah have mercy on him) said: 
This shows 
    that innovators and evildoers and those who go against the Sunnah despite 
    knowing it are to be forsaken, and that it is permissible to forsake them 
    permanently. The prohibition on forsaking someone for more than three days 
    applies to one who forsakes another for personal or worldly reasons. As for 
    innovators and their ilk, they may be forsaken permanently. This hadeeth is 
    one of those that support this view, along with similar hadeeths such as the 
    hadeeth of Ka’b ibn Maalik and others. Sharh Muslim (13/106). 
What you 
    have to do with regard to your friend is advise him, exhort him, and remind 
    him to fear the Hereafter. If he responds, then praise be to Allaah, and you 
    will have the reward for that. If he persists secretly in his sin, then he 
    deserves to be forsaken by you if you think that forsaking him will benefit 
    him. If forsaking him will not benefit him, then we do not think that you 
    should forsake him, rather we think that you should still keep company with 
    him in the hope that that will benefit him. 
Shaykh 
    al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: 
This 
    forsaking varies according to whether the forsakers are strong or weak, many 
    or few. If the aim is to rebuke the person, discipline him and deter the 
    common folk from doing something like him, if the interest is clear in this 
    case so that forsaking him will weaken evil or make evil disappear 
    altogether, then it is prescribed. If neither the person who is forsaken nor 
    anyone else will be deterred by that, rather the evil will increase, and the 
    one  who wants to forsake him is weak, and the evil consequences of that 
    will outweigh the good, then it is not prescribed to forsake him, rather 
    kindness towards some people will be more beneficial than forsaking them. 
And 
    forsaking some people is more beneficial than being kind to them. Hence the 
    Prophet  (peace and blessings of Allaah be upon him) was kind to some 
    people and forsook others. Majmoo’ al-Fataawa (28/206). 
Finally, we 
    would like to point out that it is possible to advise him and guide him to 
    goodness without confronting him with what he has done, especially his 
    taking part in the chat rooms. It is also possible to advise him and send 
    him a letter without him knowing who you are, if that will reduce his 
    involvement in committing sin openly. 
May Allaah 
    guide us and you to that which He loves and which pleases Him, and make us 
    all doers of good and resisters of evil. 
And Allaah 
    knows best.