I have to agree with your comparison of GFA and HBasic, since I gave both a try. Although at the same time I respect his choice of trying to reuse many years of built up code. It might be trivial to switch to GFA, well I think it is as far as learning goes. But moving lots of code might be rather tedious. When I got fed up with the HBasic GUI and it's issues with MiNT I sold it. I do not have a proper manual. If I had a manual it would be possible to create a HBasic import module for the newer GFA editor and let it port the bulk of the code. Then it would only be a matter of tweaking and fixing smaller difference.
He may end up switching to something else anyway if HBasic does not cooperate with MiNT as it did for me. Rebooting to single TOS just to code drove me nuts. I only tolerated it for about a week or so tops. ;o)