Most Helpful Girl
In this situation, you don’t. The person being cheated on doesn’t want to see it, and it’s between them and their partner. Unless you have proof that you have seen yourself and can give to them all you can do is support your friend if they need it.
Most Helpful Guy
Sometimes it just comes down to hard evidence. I had a friend who was engaged to a girl who cheated on him with NCOs, officers, and numerous other guys. He was afraid she was cheating on him, but she would always tell him that she wasn't. It wasn't until multiple people showed him photos of her with a marine that he finally ended it.