I think my girlfriend is being brainwashed by another guy?

ive seen some recent texts from a guy that likes my girl... she says hes only a friend.. but the texts read.."single yet" and "i still feel bad about what happened... i shouldve been more respectful and kept my distance" they hung out on our 7th month anniversary, as i was out of town. and everytime he texts her she hides her phone while texting back... im getting suspicious.. i brought it up a few nights ago and she said she likes the wa he talks to her? what should i do.. and no i will not break up.