Wtf do I do??

Okay so there's this girl I worried about with my boyfriend. They use to be best friends and me and her had a falling out but were civil and he stood up for me. I found her texting him calling him nicknames and puts kisses and I've looked and he's liked all her photos and ones where you can see her tits a little and I'm scared. People say guys prefer beunettes over blondes and I'm blonde and she's a brunette. What do I do? Do I mention this to him or what??? Help!!