She married him after he became Muslim and now he does not pray
Question

                My sister married an American man who she said was a Muslim. He is an officer in the American army in one of the Arab countries. They stayed with us for a week and I did not see him pray. I advised my sister to pay attention to this matter so that her marriage would be valid but unfortunately she did not pay any attention to it and neither did he. Is this marriage valid according to sharee’ah? 

His work is against the Muslims – is that permissible?.
Praise be to Allah.Prayer is 
    the most important pillar of Islam after the Shahaadatayn. Not praying is 
    major kufr that puts one beyond the pale of Islam. The evidence for this has 
    been explained in the answer to question no.
    5208. 
It is not 
    permissible for a Muslim woman to marry a man who does not pray, and it is 
    not permissible for her to remain married to a man who stops praying after 
    the marriage contract has been done. 
As this man 
    has recently become Muslim, it seems most likely that he does not know the 
    rulings of Islam or the importance of prayer, or that not praying is kufr.  
So it is 
    essential to explain to him the status of prayer in Islam, and that not 
    praying constitutes kufr, and that Islam and not praying cannot be combined. 
    If he starts to pray then praise be to Allaah, but if he persists in not 
    praying then he and his wife must be separated because he is not a Muslim. 
Secondly: 
It is very 
    strange that your sister did not pay attention to this matter after you 
    advised her. The relationship between them may not be proper according to 
    sharee’ah, and we do not think that she would agree to that. You have to 
    carry on advising her. 
What we are 
    afraid of is that your sister may not be praying either, which would make 
    the matter more serious and more complicated. 
Thirdly: 
With regard 
    to working for or joining the American army, the ruling on that has been 
    discussed in the answer to question no.
    3885.
And Allaah 
    knows best.