Sounds like the fiberglass created a stress point. If the fiberglass hadn't been there then the stab may have been able to flex rather than break. But this is just Monday morning quartebacking.

You could use a 1/4" spruce or basswood spar in the middle of the stab.

The strongest stabs are built up and sheeted. It's more work, but they're stronger and lighter.

What was the plane doing when it folded? Was it a high-G pullout from a power dive or something?