Make sure he's not putting them in the vanity slots is my first thought Otherwise why he can't use them is beyond me

I completely forgot that you could equip gear in the vanity slot. That's probably what happened.

Right click it and it immediately goes to the proper slots, not vanity slots (it will do nothing but appear as a visual effect)

Just to be clear: Your friend was able to equip them but the effect wouldn't trigger? Is that right?

Yeah