Bf "cheating" psychologically

So my bf and I have been dating for 8months now. We met in college (2014) and have been a "thing" since February last year. And today I found out that he was talking to some girl (that knew that I exist) for a week know. They were talking, among other things, how hot she was and how they wished they had sex. They even exchanged nudes on Snapchat... I will talk to him on Saturday, because it's when my college exams end. But I don't know how to talk to him, and whether or not I should break up with him...

I wasn't expecting this because he is super sweet with me and my two bff said the same thing, that they didn't saw this coming...

It's this really cheating or am I overreacting?