Friend's boyfriend still texting his ex (that he cheated on)...thoughts?

My friend told me the other day that she saw on her boyfriend’s phone that he’s been texting his ex-gf.

So she asked me if she’s over-reacting by not being comfortable with the fact that her bf still texts his ex. I don’t know what to say since there’s not a whole lot of info to go off of; I don’t know how long they’ve been talking, what they talk about, etc. and frankly I know it’s not my business. She did ask him about it, and he said something to the effect of, “you don’t act like this when I text my other friends”.

They (my friend and her bf) got together in the first place because he cheated on his gf (the ex in question) with her (while she knew he was already in a relationship). So I want to say she’s feeling insecure b/c of the way she got in her relationship in the first place, and now she’s paranoid that his feelings for the ex may come back (They’d been together for 3 yrs before he cheated, so…). I’m curious what other people’s thoughts are.