I never understood how couples seem to go from lovey-dovey to hating each other and not on speaking terms in a snap of the fingers.
I would be perfectly happy to meet my ex for a coffee, he's just one of my friends after all.
I certainly wouldn't ask my current partner if he was OK with it. Would probably tell him about it either before or after though.
As far as I'm concerned, if a person can't handle their partner meeting up with one of their friends for a coffee, ex or not, then something is wrong, or they have a lot of maturing still to do.