"Alternatively or in addition, I'd also have some inclination to tell a parent."

I would be very cautious about this. While some parents are reasonably enlightened, most are not - and in either case telling a parent would be violating the trust the girl placed in you. While there might possibly be a situation where this was required, generally speaking no good could come of it. Explain to the girl for yourself the reasons why you cannot take her up on her offer.