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