I would still keep quiet, but you might want to consider the best value of either situation to your mate.
I'm assuming it is clear that he knows of the boyfriend, so it's not news to him that she was cheating on him. With the legal situation does it matter how many guys she's been with to the outcome of the settlement? My guess is probably not, but I don't know the law of the country your mate is in.
On the other hand if the information is material and the money he gives her would be significantly less, you might want to think about telling him and explaining your dilemma of not telling him. He should take the welcoming information and will at least have more money to blow on punting.
You could always make up a story like you were talking with your brother and you were telling him that your mate is getting divorced, he asked who and you said so you told him that he actually meet the wife one time on her vacation...etc. Just hope he doesn't blame your brother for starting her cheating ways.
Hope things work out for the best and quickly for your mate.