Can one give a gift to a friend for buying a house with a riba-based loan?
Question

                I would like to ask about the ruling on giving a gift to a person who has bought a house by taking a riba-based loan, knowing that I advised this person before he bought the house, but he insisted on doing it. Is it permissible to accept his invitation to celebrate his buying a new house and to give him a house-warming gift?.
Praise be to Allah.If the matter is as 
    described, and he has committed this grave sin and insisted on it, despite 
    your advice to him, then you should not accept his invitation to celebrate 
    his buying this house, or give him anything on this occasion, when in fact 
    he needs to be offered condolences, not gifts, because riba is a major sin. 
    Allaah says (interpretation of the meaning): 
“Allaah will destroy 
    Riba and will give increase for Sadaqaat (deeds of charity, alms). And 
    Allaah likes not the disbelievers, sinners”
[al-Baqarah] 
“O you who believe! Fear 
    Allaah and give up what remains (due to you) from Ribaa (from now onward) if 
    you are (really) believers.
279. And if you do not 
    do it, then take a notice of war from Allaah and His Messenger but if you 
    repent, you shall have your capital sums. Deal not unjustly (by asking more 
    than your capital sums), and you shall not be dealt with unjustly (by 
    receiving less than your capital sums)”
[al-Baqarah 2:278-279]
And Muslim (1598) narrated 
    that Jaabir (may Allaah be pleased with him) said: The Messenger of Allaah 
     (peace and blessings of Allaah be upon him) cursed the one who consumes 
    riba and the one who pays it, the one who writes it down and the two who 
    witness it, and he said: they are all the same. 
And the Prophet  (peace 
    and blessings of Allaah be upon him) said: “A dirham of riba that a man 
    consumes knowingly is worse before Allaah than thirty-six acts of zina.” 
    Narrated by Ahmad and al-Tabaraani; classed as saheeh by al-Albaani in 
    Saheeh al-Jaami’, no. 3375. 
You should carry on 
    advising him to repent to Allaah from what he has done. 
We ask Allaah to forgive us 
    and him, and to set the affairs of all the Muslims straight. 
And Allaah knows best.