First off, I want to say that opinions about this of course vary widely and that my take on this probably shouldn’t be the one you listen to. I will give you my opinion though, having loved and lost a lot — both romantically and platonically — in my short life.
To put it simply, it depends on the circumstance. If you’re legally underage, chances are your relationship won’t work out for the rest of your life. Thus, I don’t really think it’s fair to call eternal dibs on someone just because you had your first kiss in the back of a Honda Civic with them, you know?
The one caveat I have to this is that first loves are always off the market. Every true friend can identify the other’s first real love, and that person is, in my opinion, the only one that deserves the permanent “hands off” sticker.
As regular adult humans, however, I think the rules change a bit. I think a good metric to judge by is whether that ex of your particular friend knows his or her (the friend’s) parents better than you do. If the answer is yes, you probably shouldn’t hook up with them.
All in all, I wouldn’t want to borrow trouble by doing that. There are eight billion people on this big rock, and we don’t have to succumb to our friend’s ex’s advances. We just don’t. It might kinda suck for like two days, but would it really be worth losing that friendship? You know, maybe it would be. Idk. This was all pointless. Mazel tov!