I think he knew once it was over. Hence buying her the dress (I was always grateful for that graceful note of farewell which the show gave them, if they had to break it up). Also, he must have had at least an inkling that it was more than just groping, considering he told her pretty confidential stuff like the fact he slept outside during Christmas because he couldn't stand being around his drunken parents then. (Some heavy ammunition he gave her.)

Apron strings: probably not, but then, later Cordy had them, too, with Angel, for whom she dropped her friendship with Wesley like a hot potato in late season 3. And considering that Xander liked Angel less at the best of times than Cordy liked Buffy and Willow, and so would have voted for staking him in season 2, I'd say they'd be even with the ties.

More seriously: have you read honorhs story about Xander being the one to call Angel about Joyce? That one contains some lovely Cordelia references, specifically, Angel asking Xander how to get back into her good graces (as timeline-wise, Body and Epiphany happened simultanously, remember, and Xander confiding about the magic of wardrobe buying.*g*