This question resolves around Quest: Fortune and Glory within the LOR expansion.

The overlords objective is to kill a prisoner who can be picked up and carried out of the exit by a hero character. When the hero carrying the prisoner is defeated, the prisoner token is placed on the board in an adjacent space. The overlord is then able to attack the prisoner (health 5x #of hero's with 1 grey defence die)

I'm having a dispute whether I can use overlord cards to add abilities/damage etc against this prisoner. Examples would be OL cards such as fury, frenzy, mental error, dirty fighting expert blow etc.

Because nearly every OL card is phrased as "when attacking a hero", my opponent states that because the prisoner isn't a hero, no OL cards can be used on it, and only dice-result attacks.

I'd appreciate some input from other fans/players regarding this. The Errata states OL cards/abilities targeting familiars labelled as "figures" are subjected to any effects of OL cards/abilities. Now the prisoner is of course a figure, but the Errata only specifically mentions familiars and not figures, which isn't unexpected as that would be an incredibly vague addition.

Thanks for your help

I am not sure of the quest.

Does it state that the prisoner is 'treated as a hero'?

In some quests this is stated for other characters - that helps to clarify such considerations.

It depends entirely on whether the figure in question is "treated as a hero." As clarified in the latest FAQ, the same rules apply to these things as familiars treated as figures. That is, hero abilities, attacks, monster abilities, and OL cards that target a 'hero' can target these figures.