To message the ‘other woman’ or not?

So my boyfriend and I have been together going on 3 years. We broke up about 18 months ago because I found out he was on tinder, but stupidly I decided to give him another chance. Since then he seems to have been trying really hard - he’s been so attentive, helpful and affectionate and I thought maybe he had learnt and changed for the better.

But the ‘curiosity killed the cat’ saying certainly holds true, and the other day when I had a chance to go through his phone I decided to take it - I know I shouldn’t have but that’s not what this is about at this point.

What I found was that during the last school holidays (he is a high school teacher), he met up with another girl for some drinks. From the messages exchanged between them it’s intimated that they’ve probably had sex before (he asked her if she preferred his cock over others), but apart from that I have no proof they actually have. He also told her he could ‘sneak something in on the Friday’ if she wanted to have sex, but she said it felt wrong and she had plans so that never went ahead.

I have a few mutual friends with this girl and have been tempted to reach out to her as I would really love to know the full story before I confront him about it. I don’t blame her as he was the one coming into her and she didn’t seem hugely interested in most of the messages.

I guess I’m just after some advice on a) whether it’s a terrible idea to reach out to her, b) if anyone else has been in a similar boat what they have done and c) what I should say if I do go ahead and message her?

TL;DR: my boyfriend secretly met up with a girl and sent her messages wanting to sleep with her. She didn’t seem fully into it. Do I message her to try get the full story before I confront him or avoid potentially turning this into a bigger drama?