Alright I tested the latest version and it fixed it with some helmets such as advanced rocketry helmets but it broke other helmets such as the avaritia infinity helmet. I haven't tested with other helmets but I'm pretty sure that you should fix it with avaritia just to be safe.

Tested with blood magic armor and it does give those helmets damage so yea not just helmets that you wouldn't be expected to ever get in a legit survival world (at least a sane person's survival world)

Honestly I've been checking this and I cannot get around it. Sorry :( the helmets for those mods should really return negative for when I check if the item is damageable; but instead it somehow results in a positive... I added another check in the latest version. Let's hope that helps.

I DID fix the bug where the helmets would instantly disappear though :)

Why do these helmets circumvent durability?.... jeez, this seems almost more a problem on their end really. Will see what I can do.

Thanks for the report btw ;P

You need to wrap this call in something like

if (stack.getItem() instanceof ISpecialArmor)
    ((ISpecialArmor) stack.getItem()).damageArmor(...);
else
    stack.damageItem(...);

Thanks for the clarification @TehNut