Over or under reacting?

Ok so my bf went to lunch with a female friend of his a couple days ago. Normally I don't care, I trust him completely. But he knows I don't like her one bit and he didn't tell me about it. If he would have told me then it wouldn't be a big deal. I saw it on his phone responding to a text for him while he was driving. She texted him first but he asked her to dinner (on a day he knows I work late) then to lunch when she said she couldn't do dinner. It really bothers me because he didn't mention it or anything. How do I talk to him about it without sounding like a jealous girlfriend? Like I said I do trust him but I don't trust her. He is a very polite gentleman and very likeable but she is extremely flirty and he confided in me a couple days ago that he use to think about cheating and has for most of his life but never followed thru because he's not the cheating kind and he knows that it hurts but he's worried he even thinks about it. I don't know what to do. Should I talk to him about it or just let it go? And if I talk about how do I bring it up?