It isn't that what she's doing is okay (it's not, especially if it is affecting everyone else in the house), BUT the problem here is that it is not your house.

Her parents control it, and they get to decide who lives there. I know you are grateful that they offered you and your SO a chance to live there any pay down your debts, but that leaves you with a choice.

You can either accept the situation that exists (reasonable rent but crazy druggie in the house) or make some other arrangements.

Sorry if it sounds harsh compared to the other comments but the facts of life are what they are. I'd recommend getting out of there as soon as you can - it may take longer to pay off your debts but at least you'll have some peace of mind.