Only a lawyer could tell you and they would need to know more about the details to properly advise you. They might have no legal claim but if they were to sue you and you did not respond then you could get royally screwed.

I would start by simply asking them if they believe there is a legal agreement between the three of you and why. If they believe there is, and that it binds you to provide software then what are their legal obligations to you.

In the event they respond, I imagine your actual lawyer (who you will engage) will want to know this anyways.