What should I do with a father who prays at home while the mosque is close by? Should I talk to him? Would I be sinful if I left him at ease, knowing that I have already spoken to him but he did not respond?

If a father prays in congregation at home, he has fulfilled the obligation according to those who deem congregational prayer obligatory. If he prays alone, he is sinful according to those who deem it obligatory. The father should be advised gently and kindly to pray in the mosque and reminded of the virtue of frequenting mosques, while utilizing indirect methods such as lectures and pamphlets. If the father follows an opinion that does not consider congregational prayer obligatory, then he is not sinful. However, the desirability and virtue of going to mosques should be explained to him in a gentle and kind manner.