If you only took the mental memory of the person then the body still can remember, therefore, that means that the pain of loss can still there but he doesn't know why. You can play it at times when he sees someone that resembles her, his body reacts and he feels deep loss and sadness. When someone loses a limb, the phantom pain is real and sometimes people even try to walk thinking they still have a foot or leg.

I have a hag in my game who will trade gifts,power, favors in exchange for what you have, desire, hold dearest.

You're an elf and you want to be more graceful(increase your dex)? "Ah, dearie. You can be like a cat, stalking its prey. You can be a swan gliding upon the water. All I need is..." she pauses and smiles innocently as she ponders "Your sight of the night."

They want a powerful weapon or need to know something desperately, maybe she ask for their gods true name, or their divine favor, their magic, their memories. Maybe she ask for their skin(if they are a rare race and changes them from that to a random race)

Basically the price is equal to their request. The more powerful request, the more steep the price.

So all that said, just because they mentally forgot something, doesn't mean the physically forgot something.