Would you tell your best friend if...?

Well my best friend's boyfriend went on a mission trip a week ago, and my other friend texted me telling me that she saw him flirting and cuddling on other girls and getting their numbers and stuff, but when she confronted him, he said it didn't matter because he would never see them again. My best friend still doesn't know because she is on vacation at the moment, and if I'm going to tell her, I want it to be in person. I'm just wondering if I should tell her or not. Her 11 month was yesterday, and I just don't want her to get really upset or anything. But then again, if I were her then I would want to know, but IDK if that's just me. Should I tell her?