I actually don't see 900$ as unreasonable, especially if they're quoting for a new bumper and paint, and assuming they're not going to be able to buff it out and paint it.
Is there any reason you can't just go through your insurance instead? You don't need a police report to do so.
If not, what about telling her to fix it herself, and then offering to make payments that you can afford? If you go this route I'd put a contract of sorts in writing and see if you can even get it notorized, that way if she tries to sue you have the contract to prove you are doing the right thing.