Are they paying you a maintenance fee? if so then you probably should eat the refox cost and not even mention it. If not then they're expecting a lot if you're supposed to maintain sourcecode indefinitely for no $ and be ready to resume work for an hour or two using a development version you and Microsoft shelved years ago. You could apologize that you don't work with VFP5 any more and would need to lift the whole system to VFP9 which will take more than an hour, unfortunately. Also you might ask them for a regular fee so you can keep the sourcecode in a form that will allow efficient maintenance if necessary.

However, refox does more then decrypt. It also compresses and encrypts so nobody else can pick your work apart. The compression is worth a lot IMHO. Also encryption is good if you're selling licenses for a standard product: Refox may not 100% prevent people pirating but it does make it more trouble than it's worth in most cases for people to delve around in your good work. So maybe this is a good time to consider refox anyway.