Fight with husband

Okay so my husband and I got in argument over a “friend”(girl) he’s talking to…now he hasn’t see this girl since she was 5yrs old and she randomly pop back into his life few months ago and then stop talking to him and just messaged him about two weeks ago. My problem is he’s been talking to her non stop since she messaged him and even told him to make a Snapchat because no one texts anymore(he never does this with any of friends he hasn’t spoken to in awhile)... I can’t ever get him to have a conversation with me but he’s having full on non stop conversation with her and he doesn’t see the problem...on top of that I’ve noticed when she Snapchat him he covers his phone. I just need some advice on this if I’m just over thinking it or what but it’s starting to bother me because he never acts this way with any of other friends