How to tell your best friend that his wife is cheating on him ?

There's no easy way to say this kind of thing. You can wait until his project is over but then there might be something else that comes up afterwards.

It's better to do it sooner rather than later though and be done with it. Make sure there's no one around when you tell him the news and be serious at all times. Mention that you're telling him this because he's your best friend and that you don't want to hide things from him.

Good luck.

If he is working on a huge project all his attention is required elsewhere and he is stressed, telling him might makes him unable to focus on his work. My advise: wait for the project to be over and tell him. But, you should have proofs and let him understand that you are here for him no matter what

Put it to him straight

i think that friend should gather enough proofs and show it to him... thats the best way i think to avoid conflict btw friends and to strengthen your point

Say it bluntly : Look pal , this project is very important and we don't have much time left to finish it . So concentrate on that and don't worry about home , your wife can get along fine without you .