She's paid up to the end of last month OP, so has paid what you've paid for.
She will absolutely not agree to pay the other £11 until you are due to pay the credit card, I think that much is clear from what she's said previously.
I'd probably let HR know how you've got on and continue taking lunch orders from everyone who has fully paid for everything that they've eaten.
I know you say that your colleague should have the funds but there are lots of reasons why she might not have them immediately available including financial abuse from a partner. She should still deal with it better but it can be humiliating to admit to some things.
If it was me I wouldn't order her any more but I would encourage people to be kind to her, if nothing else this would be a good look for you.