I don't understand why, if she doesn't pay, no one can have lunch?
Surely the obvious answer is if you want lunch, you pay in advance, if you don't pay, you don't get lunch? No need to penalise the rest of the team.
Of course in B's case, it's pay the remaining 11 quid and up front for this week and you can join, otherwise no lunch for you until all is paid.
But your other colleagues still get lunch regardless of what B is doing...