Okay so I was watching an old-ish movie called The Dilemma (Vince Vaughn and Kevin James)

Vince Vaughn discovers that Kevin Jame's character is being cheated on by his wife, but Kevin James is under huge pressure to complete a work project with a lot of money riding on it.

His dilemma is does he hold in the secret until after the project has been completed or does he tell him and risk their company going bust and the project failing, as it would obviously affect his performance.

So would you tell your best friend before or after?

Ooh, that is tough.  I think you gotta tell him asap.  Otherwise, it'll ruin the friendship which is more important than the money.

I would say after. His grief over losing his girlfriend is one thing, his grief over losing his girlfriend, his own company and putting his bestfriend out of a job is a whole new level of grief.

I've been in a similar situation :c wasn't cheating but somehow similar to this

Try your best to show no impressions of you knowing anything, wait until project is done & is successful
By that time you would have figured out the best way out of this with least damage

After that you tell him about it, show him options he has and let him decide

There's only one problem that I have encountered when I was in a similar situation, what would HE do if he found out about it, and also found out that YOU knew and refused to tell him.

i always put friendships first they are far more important then money....which would hurt more losing money or losing the trust and respect of a good friend its hardly a dilemma for me friends come first

If you explain to your friend why you waited to tell him, I think the friend would be understanding. If it were me, I probably wouldn't even reflect much at all on the fact that the friend withheld the information for some time. In a situation like that, I believe the unfaithfulness will consume all your emotions and there simply isn't room for disappointment in the friend. 

It depends on the relationship to be honest. If it seems like an overall unhealthy relationship for your friend to be in I think you gotta tell him. Otherwise I let it go till after the project.

I definitely would wait  until after, and explain why.  The friend would appreciate this,.

personally i would tell my friend before hand but help them finish their project......idk cause i feel after they will be uber happy they finished this and then to drop sad news on them will just ruin that entire moment. so if i tell him before hand help him finish project he will get his stuff done and have a happy moment after you know? idk i feel that would be easier.