What do I do? She is threatening my relationship!

So my BF of 8 years got a message from a girl from his past on Instagram. They went on 1 date before we met. She told him she has wondered what it could have been between them. He responded truthfully and said he had also wondered but shut her down saying he was in a relationship…months pass and they connect again and go for a hike together (we are really outdoorsy) they had a great time and he was hoping they were on the same page of being friends from their last conversation. (All in text that he showed/read to me) Afterwards she admits she has feelings for him but down to be friends…he is interested in her but not interested in pursuing her. I’m just at a loss of how to handle this. I really appreciate how honest and transparent he has been with me but I don’t love where this situation is at obviously.

Can guys and girls really just be friends when they find each other attractive?

How do I play this cool? I don’t want to be that psycho GF and make matters worse.

I really do trust him…I just don’t trust her. Without knowing her I feel like her intentions are potentially malicious, but maybe she is just human…idk

Please…help me feel better about this! I’ve been sick to my stomach, my past is absolutely triggering me right now.