Considering letting her scratch an itch

I am in quite a unique scenario. My wife was married for 20 years before we got together. Their relationship deteriorated as she became aware of his narcissistic and controlling nature. He became caustic, manipulative and physically and emotionally abusive. During that time, her and a great friend of hers from her work as nurses developed a relationship that was supportive of each other and they commiserated about both of their troubled marriages over two decades. They began to develop feelings for each other. Both were married, and were not of the mindset to engage in infidelity. But they both wished they could be together and overtime fell in love. Lots of flirting and tongue in cheek humor. They definitely had the hots for each other, But there was no physical relationship. Both fantasized about leaving and getting together. Typical unrequited love story. Wife finally divorced her ex. Her "work husband" he didn't leave and not long thereafter, his wife got cancer. He chose to stay and took care of her. She moved from up north to another state and about a year later Now-wife and I began dating. Actually feel sorry for that poor guy. I’ve met him and he’s a stand-up guy. If the timing hadn’t been off, they definitely would’ve gotten together. Anyway, wife was very forthcoming about the nature of their relationship, but she felt like when she left her husband he had an opportunity to do the same prior to the onset of his wife’s illness and he chose not to, that he had made his choice. Then he was stuck. About three years into me and wife dating and about six months prior to us getting engaged his wife passed away. So now he's single. They have stayed in touch and there is still a flirty nature to their interactions. I tease her about it a lot. I confessed I thought is was hot thinking about her having a hall pass and having sex with him. We have roll played a bit around the idea and it's been super hot. anyway, as I was saying about six months prior to our engagement, wife went back up north to help one of her girlfriends who was having surgery. Stayed up there for a week helping her friend through her convalescence. During that time And her grab dinner at a nice Italian restaurant. He was flirty. She was flirty. He asked her if she was sure about getting married, and in roundabout language made it clear that he would still love to get together with her if she wasn't sure. She told him she was sure and that she was in love with me. they didn’t do anything. Not even a kiss. However, she was not unaffected by his overtures. She came back horny as hell. We fucked for three days role-playing and fantasizing about him being our third. Or me walking in and catching the two of them together.... or her being in former city and FaceTimeing me while she indulged herself and this decades-long fantasy. Well, long story long, i’m thinking about giving her an actual hall pass. She has said that she really doesn’t have a burning desire to be with him or have closure, but I’m not sure I believe it. I think she might be saying those things in order to guard my feelings. I trust her completely, and I almost want her to get it out of her system so to speak. Plus, I think it would be super hot. Coming here for advice. What do you think about this situation? Too dangerous to allow, or is it something that will always be in the back of my mind if I don’t let her get it out of her system? Thanks