I understand that. When you have really strong chemistry with someone, it doesn't really matter how objectively unattractive they are - being near them can still totally undo all your common sense.
For this reason I think I would really struggle to be "just" friends with FWB. Whenever I see him I just want to jump his bones, and we always end up in bed together very shortly after he arrives.....
It sounds like you have similar with Tree. So if you meet him in person, it's possible you will just swallow any nonsense excuse he comes up with, because your desire will be stronger than your reason (IYSWIM). I may be projecting here, but I feel like that's the situation, from what you have described?
So, I have been thinking about this. The least worst thing that has happened is that she is just a friend, they dated, didn't work out for whatever reason but stayed good friends, and he didn't tell you about her because he felt the friendship would unsettled you, make you feel jealous or whatever, and he likes you too much to rock the boat. Maybe he kept meaning to tell you, and as time went on it just became harder.....
IF this is the case (and this is the kindest possible reading of his actions) and if he could prove to you they were purely platonic (maybe show messages between them, or even ask her to message you).... Would you accept this? Is it something you could get over?
If so, it might be worth asking for a meeting after his return, to get it all out in the open.
If you couldn't forgive him (and nobody could blame you for this! Even if they aren't shagging, it's a big deception) then in some ways maybe it's better to never see him again ....
But you may then be always tortured by wondering. I have things like this, where I walked away but would now literally give a kidney (one I had stolen from a date obviously, not my own 😉) to know the truth of certain situations. There were certain occasions with FWB for a sample, when we were still supposed to be exclusive, when we rowed and I know he went to meet his best female friend and I would kill to know whether anything actually happened (I strongly suspect something, tho maybe not full shagging - she used to get pissed and darkly hint about it). It wouldn't change anything now - there are lots of more pressing reasons he and I will never make a proper relationship work - but still, the curiosity burns!
To my mind, the very least Tree Stump owes you is a full and frank explanation. But they doesn't mean you will ever get one..... You may be more likely to get the truth out of her, quite possibly. So I would keep hold of her Facebook profile, so you can contact her if you do ever decide you want to .....