I absolutely gasped when I read that B has only refunded your in part. This goes way beyond cheeky.
What on earth makes her think that it is fine for everyone else to pay on the day but she is allowed to leave it until....well, whenever she feels like it? The cheek is breathtaking - even if she firmly believes her own theory on only paying at the end of the month, common decency would surely dictate that she follows what everyone else does and pay on the day?
Sad as it is, OP, I would take HR’s advice and put a stop to lunch club. B is such a piece of work, I wouldn’t at all put it past her to raise a grievance about bullying (but I would be letting everyone know, in no uncertain terms whose fault it was that lunch club was ruined).