salamu alaikum.

1. What difference does it makes when i'm in a non muslim country, must I pray the fard prayers in community (masjid or is it not longer required because it's not a muslim country?

2. What should i do, when i have a good friend and he is married, and i'm married too, and my wife know his wife. And this friend of me, does cheating his wife with a other woman. I tell him this is not allowed, but he has still contact to her and don't listen to me. His wife don't know anything about that.

So it's allowed for me or possibly even mustahab or fard to tell his wife that he cheat her with a other woman? (of course i will say it my wife and she will tell her). What said the ulama about that what should i do? as i said he won't hearme

 Alhamdu Lillah

1- Praying in congregation is highly recommended according to the vast majority of scholars. Some even said it is a communal obligation, meaning that the Muslim community anywhere has to facilitate holding the obligatory prayers in Jamaa'h.

No difference in this regard whether a Muslim lives in a Muslim or a non-Muslim society. It could be harder to establish Jama'ah or a Masjid where Muslim can offer their prayers regularly, but this  does not waive anyone from doing his best praying with Jama'ah.

2- Preaching your friend to quit what is he doing is the first thing to start with. May be you can talk to somebody else who has an influence on your friend to talk to him too. Once you loose hope in him listening and quitting sinning, then informing his wife becomes a must, especially if your friend is having intimate relation with that girl/lady. This is because of his disqualification of being a husband for a chaste woman while he is not!