In general, I would say that yes, the class IR apply to adventurers. You cannot order, suggest, or coerce an adventurer into changing to Noble, whether they have the recommendations for it or not.
However, this seems like it might be a somewhat special case. If I understand correctly, the adventurer in question wants protection from a manhunt, and you're willing to give it on the condition that they change class.
This seems related to (though not clearly analogous to) offering compensation for those who will change unit types. It also sounds like part of the protection in this case would be the class change itself, as once they were no longer an adventurer, they could not be arrested and executed out of hand.
Can you please expand some about the situation? Is there any other particular reason you/your character wants the adventurer to become a noble?
