Surely you don't believe there is a law that says you cannot tell a former employer you don't wish to help them in an endeavor that has nothing whatsoever ( from your posting ) to do with work? What is the relationship between them asking for help and the fact that you used to work for them but got fired? You are certainly allowed to tell them whatever you wish----I don't see the correlation between the two.

I believe the distinction is perhaps between a LEGAL obligation and a moral or ethical one? While I am not certain that ANY obligation exists to help ANYONE ---there is certainly no legal one. I am interested it knowing why you would think there was even a remote chance there was. Please explain.