How can one avoid using foul language with close friends who use it? And what is the evidence for the prohibition of foul language and its ruling?

It is not permissible to use foul language, nor is it permissible to associate with those who do, even if they are your closest relatives, because "there is no obedience to a creature in disobedience to the Creator." You should advise them by reminding them of Allah's hatred for the obscene and foul-mouthed, and that obscenity is not a characteristic of believers. If they do not desist, then do not sit with them except while condemning their actions. If you are compelled to sit with them, such as in a classroom, then condemn it in your heart and avoid discussing obscenities with them. Prophetic traditions have indicated the prohibition of foul language and Allah's hatred for it, such as his (peace be upon him) saying: "Nothing is heavier in the balance of a believer on the Day of Resurrection than good character, and indeed Allah hates the obscene and the foul-mouthed," and "The believer is not a slanderer, nor a curser, nor obscene, nor foul-mouthed." Using foul language is forbidden but it does not constitute disbelief.