Out of curiosity, what 400V+ hardware is he working with that needs ESD protection?
I use 5kV rated gloves when I work with high voltage vacuum tube stuff (in ham radio hobby capacity - of course stuff is never energised, but I would rather be too safe than die by forgetting to switch something off, there are caps just in the psu with enough energy to kill instantly, they have bleed resistors, but those can fail too).
Those are the cheapest certified gloves I could find.(they were about $30 if I remember correctly). They are supposed to be single use. They come with an instruction to blow air into them to check before use. Also they have thin cotton-like gloves you put on first, then the rubber ones. It means finger dexterity is severely affected, but one is supposed to grab stuff with isolated tools anyway. Gloves are not there to allow one to grab live wires, they are there just as an extra layer of protection.