I would definitely inform my friend. That old adage about "what people don't know don't hurt them", is baloney! Even though people think they don't know, deep inside they do. Especially in a marriage. I would want to know.
True Story: A man of my friend's was cheating on his wife. He made a LOT of money in the oil fields. He was so sneaky that she could never catch him (or so he thought). He wanted her to divorce him, but not give her a settlement at all. She hired one of the best Private Investigators in the state of Alaska.
He thought he was so smart as he was cheating with a woman in another state. One morning as they were leaving her house, outside was his wife, her attorney, her 2 sisters, and the PI, all sitting in the back of a parked pickup truck out front eating sandwiches, with a video camera rolling. He bled some substantial money over that one.