Having a lot of experience with dealing with deaf people, I know all to well how the vast majority will fight you to the death about not getting the devices they actually need (hearing assist devices)! They have no problem wasting hours of your time in their delusion that their hearing is not that bad.

I'm guessing the Sennheiser headphones he has now, are just the regular consumer ones.
If your customer does have a hearing problem; there is NO line amp you are going to get, that is going to fix the issue.
Sennheiser makes 14 different models of hearing assist headphones. Here are some for $249 MSRP

[Link: en-us.sennheiser.com]