I use lightweight fillers of various kinds in the early stages of plug making, including lightweight spackle. Once the first layer of glass goes on, I stop using those "soft" fillers, and go to either acrylic auto body putty, or epoxy and microballoons. (or epoxy and talc) I've never had the glass separate from the underlying spackle either, and some of my plugs are 10 years old. (I believe that the epoxy would harden the spackle as well as CA, but be easier to sand.)

This is probably just a "quirk" of the particular chemicals your friend used, but I don't think it's a common problem.