It could have been that after two drinks with him she decided she couldn't stand his company a minute longer.
I'd have probably turned down the second drink - it normally only takes one to realise someone is an arsehole. But maybe he was insistent and she was embarrassed. It happens.
I wouldn't have offered him the cash in compensation for not shagging him because that would have been awkward and insulting.
I'd be surprised if most men would take it under those circumstances anyway. But he sounds like the kind of charmer who would.
If he'd have insisted on the money, of course I'd have paid up, and then posted on mumsnet as soon as I got in - after bolting all the doors.
I wonder if he has a coin purse?