Ow OP. I'm sorry that this has happened to you. I'm a nutcase so if I saw something like that on DP's phone I would just rage in and start yelling... Probably not the approach you want to take when he's just had surgery.
Maybe figure out what this means for you, what you need and want from the resolution (i.e. no more cheating, counselling, etc) and then go and confront him.