Good response, Panda (and thank you for the new thread, I really wanted to respond to the last effort from her and the thread was full!)
Very glad you have F2 and F3 on board. I presume you've shown them the message she sent you?
She's delusional if she thinks she can get away with "blame on both sides" - no! it's ALL hers, and if she can't accept that, then she's no kind of friend. I assume though that F2 and/or F3 have already been on her case, telling her what kind of bellend she has been; and possibly also the PTA lady has had a word about the sort of person who belongs on the PTA being rather unlike the way F1 has behaved.
Either way - when you see her, don't back down. She HAS been a bellend and a CF, you've given her more than you needed to by way of agreeing to donate anything, and asking the PTA lady to go easy on her, and she's STILL being a dick about it.
She won't apologise, it's probably caused her to grind her teeth down to stumps just to take partial blame, but only you can decide now whether you can look past that and continue to socialise with her. At least you know you can't trust her, and she's not the friend you thought she was.
Good luck tonight - hope F2 and F3 manage to prevent her from being as vile to you this time as she was last. xx 