I fought the dragon, died, came back, and the grave was broke.

Seems like other mod is ignoring Forge hooks: graves are already protected against vanilla dragon. I can't do anything more, since problem is here

Question, would turning off mobgriefing allow the grave to survive the dragon?

Looks like HEE code implements this check, so it should work.

thanks for the tip, not telling my brother though. Been bugging me like crazy about going to kill the dragon.