Boyfriend. “Old friends”

So. My boyfriend has a friend. That he’s known way longer than I have known him.

The only problem I have with this friend is that they were talking about meeting up.

Which. He does/doesn’t know I know about because I was reading the messages right in front of him. Any way. They were talking about meeting up. And I’m not really sure how I am supposed to feel about it. I don’t remember if he told me. Or not. Honestly. I don’t think he did. Cause it was only in February. I’m in need of advice on this.