Hi, things that just came to mind:
She’s been up front and honest about it, so really it’s probably not your decision?
You could ask HIM what his intentions are, and/or try to set him up with OTHER women, and see if that bait is taken or avoided like the plague.
Really sounds like you have trust issues with your lady and I would aggressively put this worry to an end so you can sleep at night, honesty is the best policy.
You could try the reverse scenario, how would she feel if you had an old flame for 3 years suddenly reappear in your life and you’d like to be friends?
You probably need to elaborate on your issue, that was a bit short, what else is there to the story?