As long as your parents "enable" her to continue making the wrong decisions financially they are in a sense getting in the way of God.
Correct me if I'm wrong, but isn't God supposed to be in control over everything? God, who is in absolute control over everything, is somehow unable to follow through with his plan because her parents intervened?
If God is in complete control over our lives, then it would be impossible for us to disrupt his plan. Even if we intervene, he should have no problem either stopping us, or adapting the plan to our actions.
Sometimes, we have to stop letting people take advantage of us. If your child is financially irresponsible and you're confident they'll wast the money you loan them, then don't loam them the money. But there's nothing stopping you from finding other ways to help them. You can always help them find work, teach them how to be responsible with money in other ways, etc.
All you're doing is shirking your responsibilities and saying, "Eh, God will handle it." You're assuming God will do all the work and all you have to do is sit on the side lines. You're assuming God has a plan, and that you're not a part of it. But let's be real here, you don't know if you're a part of it or not. So don't assume and act accordingly to what you do understand.
Sometimes, you shouldn't act when someone asks for help. But this should only be done when inaction is helping them (such as refusing to give money to someone you know is going to spend it on drugs). This is different than not helping them, it's helping them through inaction.
If God has a plan, then you're unaware of what that plan is. So instead of worrying about whether or not you'll get in the way of the plan, just do what you feel is the most helpful in the situation. You don't know the plan - then don't worry about it. Surely, God being all knowing, will adapt his plan according to your ignorance, no?