I think too many responses in here are focusing on the lie. It's the act that matters, not the cover up. When a woman does this the relationship becomes cursed. He probably went through her phone because he suspected something. Everyone is free to do as they like when they are single; however, those who live this type of lifestyle are eventually found out and have to deal with the consequences.
I've had women ask me on the first date if I was seeing someone else and then after the second tell me that if I sleep with someone else while we are talking then she is bouncing, only for them to later tell me they were sleeping with someone else or did sleep with someone else at some point when things got murky. People like that are scandalous, and there is no way to flip it. It's okay not to be naive and know this may be or probably is happening, but also it's okay not to accept it.