I think I would go the collections route too.

I don't think any way you do it you'll get your money. You could refuse care... but she will probably just leave. You could demand it, but you probably wont get it. You could develop a payment plan with her, but that may just put you farther in the hole.

I would cut my losses, demand payment in full by the end of the week otherwise no care will be provided next week and hope you can get something from her and then threaten her with collections come monday.

Best of luck to you. I can see how this would spiral out of control. This is exactly why we have payment policies and need to stick with them.