Setting aside all the practical ways this suit could be handled affordably (e.g., her actual damages were a much smaller monetary sum compared to that invoiced amount and probably eligible for small claims)...
Having a policy around cancellations in the invoices would not materially effect anything here. While it might be helpful to ensure a good-faith customer behaves in a professional and appropriate way, such policies have little effect on a bad-faith customer.
Even without an explicit policy, this is fairly straightforward promissory estoppel, or at least something very much like it. If she had a policy, she would have a very strong case. Without, I still reckon she has a very strong case -- pretty much just as strong. Either way, the recourse is the courts.