First, why do you think it will be a bad letter?
Second, I'd vote ask. My mother taught at a college and was upfront. She'd tell someone she'd write a letter but it wouldn't be a good one, and they'd usually withdraw the request. N=1, but at least there's a chance of finding out, while there's none with not asking. And if she gets offended by the asking, then she probably doesn't like you anyways and wouldn't have written a great letter anyways. Unless you plan to continue working with her, I doubt you're burning any bridges here.