6
Singular they rule
(pawb.social)
Be sure to follow the rule before you head out.
Rule: You must post before you leave.
Does it? If multiple people left multiple wallets on the table, it would be, "They left their wallets on the table." Multiple people can't really leave a single wallet behind. Or at least that would be very unusual and unintuitive.
True. I'll change it to a (hopefully) better example.
This sounds normal to me, how else would you word it?
If it was a singular customer, I would say "He forgot to pay his bill" (or she/her depending on the gender).