I found this neat post at The Soulful Spike Society
, said by Rob:
"I've always believed Buffy is well aware of Xander's actions in "Becoming Part 2." She had an entire summer alone in LA to reflect upon every moment of those fateful days...and it didn't require Columbo to work out what had happened. Three reasons why Buffy knows:
1. Xander - after a hesitation - quotes Willow saying "kick his ass." Really now, who can imagine Willow using that sort of phrasing, especially at that time in the series? Buffy knows her better than that. More importantly, there is no possibility Willow would be that insensitive to Buffy's feelings under those particular circumstances.
2. Xander's not the most convincing of liars. He can do it, of course...but if Buffy had truly been paying attention, she knows him well enough to have spotted it. Truthfully, she was barely paying attention to him, totally in gameface at that moment.
3. Most obviously of all, Angel returns (far too late to make a difference in the end, I might add), making it patently obvious Willow was re-attempting the mojo. Buffy tells us she is aware of this in Season 3 when she informs Willow and Giles that the spell worked.
Note that Xander was NOT in the library when she shared those specific circumstances. That, to me, is very telling. She wasn't ready for him to be around when discussing exactly what happened.
Based on the above points, I firmly believe that over that summer in LA, Buffy chose to understand and forgive what Xander had done. She knew how lackadasically she'd fought Angelus in "Becoming Part 1," and probably realized he'd have killed her if Xander had delivered that message accurately.
Is all this enormous fanwankery? Well...sure. But it's fanwankery with all sorts of logic to back it up...and it's a hell of a lot better than Joss chose to give us, which was basically nothing.
Assuming all my theories are correct, Buffy forgave Xander...but forgot? No chance. Like most families, she tucked it away in her mind for some time in the future when she'd feel cornered.
The argument in "Selfless" was the perfect time to pull it out of mental storage, and it hit Xander - not to mention every hardcore fan - like a sledgehammer. At that moment, the argument was effectively over."