BUT, other things besides "abilities" do actually proc this set.

I wore Infernal Guardian while tanking a normal non-dlc dungeon (just for fun) and I noticed that it was randomly proccing for no apparent reason.
I believe it was one of these or maybe all of them:

Champion Point (red) Steed tree - 120 points - "Reinforced" when you block gain a tiny damage shield
or
Psajic Skill line passive "Concentrated Barrier" - when you have a Psajic skill slotted and you block, you get a damage shield
or
Destruction Staff skill line passive - "Tri-focus" - with a Frost Staff equipped, heavy attack grants a damage shield

I don't think I was using a Frost Staff and I probably did not slot a Psajic skill either, so I believe it must have been the CP passive "Reinforced".
Another thing that "used to" proc it was a damage shield weapon enchantment. I believe this still works.

Note: never tested the set Lunar Bastion (drops in Maw of Lorkaw) but it would be nifty if the little damage shield granted to the whole group could proc Infernal Guardian worn by anyone in the group. Just for fun.

Hope this helps.

FYI, Infernal Guardian can occasionally be a nuisance to tanks. In things like VAA, it can aggro the axes and pull them into the group. But more often, it can start a boss fight in a trial inadvertently by raining meteors down before the group gets set. Not saying don't use the set, but be aware of some situations where you might not want to use it or at least hold off on applying your shields until the fight begins.

Nah, I wouldn't take her into a big thing like that. She was made to be a solo survivalist. Take on World bosses solo with enough dps to pass. I never expected her to be able to tank a Vet dungeon so well.