if I were you, I would start to deconstruct the thinking behind the jealousy and worries. I would recommend asking yourself why it would be okay with you for your wife to be sexual with another man without emotions, and yet it makes you uncomfortable that they have developed feelings for each other. What are the thoughts that come up when you feel that way?
You've been together a long time, so my guess is there may be some sense of possessiveness or ownership on your part. I think this is a common element I see in situations such as yours. Perhaps something in you feels like she belongs to you, in a way, and so threesomes were easy and non-threatening because you were there and able to control access to her. Then you thought of this guy as providing a service to her while you were traveling, so you allowed them to have liaisons without you. Somewhere in the mix, you were overlooking their feelings, humanity, and ability to connect.
It sounds like you are much more aware, open-minded, and willing to get to the bottom of this than some husbands seem to be when they come here with similar problems, so kudos to you. I would say, think about it this way: doesn't your wife deserve all the lovingness in her life that comes her way? Doesn't she deserve another person who cares about her and will treat her with love and respect, rather than just being a body for someone to use to get off? Because sharing her just for sex kind of demotes her to be less than who she is (a body), but allowing room in your relationship for her to feel love and be loved lets her become more fully who she is (body, mind, spirit, emotions). That might be something to remind yourself of when feelings of jealousy arise.
As for worries about her leaving you, just keep communicating. Yes, develop a friendship with this guy. It kind of doesn't make sense that a person who is happy with two partners would want to choose one; she's got a great thing, so why would she change it, LOL.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein