When a customer shows up with an unfamiliar format you invest the time to become familiar with it - this may involve buying test tapes (that you don't mind damaging) so you can test your recovery process on it and make sure it works before running it on the customer's tape.
Unless you are clearly making the customer aware that you are doing so, I think it's slimey to do so. It's very "fake it till you make it" to advertise a service that you only assume you can pick up as needed.
