Not really that big of a caveat. Only the weapon will affect enmity, so they just need 1 lucky ST run for a tome/sand and it's all good; whether the rest of the set is full weathered, full AF, or full dreadwyrm is entirely irrelevant for enmity purposes. Hardly takes a month of no life-ing to run ST. Flash doesn't scale with the weapon anyways, so overgearing is more of a problem for single-target. But for single target you get crazy enmity modifiers, so that's not a problem either unless he's trying to hold hate from an i130 MNK. So it's not really a problem unless the tank just isn't playing their class properly, in which case gear wouldn't help much.