You don't owe them anything if you were upfront with the product you were selling, which it seems you were. This honestly is not your problem, and it seems you've already gone beyond what you owe this person. I would maybe direct them to some online resources that could help them, but I agree with you that once you hold the private keys to their coins this will become more complicated for you.
Exactly. I keep explaining to them that as long as they have their 24 words, that they have not lost their coins and that they would eventually be able to regain control with another wallet.
What struck me as odd is that they kept going on about losing coins from a hardfork, when there have been no hardforks since the sale of the wallet. When I pointed that out they quit mentioning it. Which lead me to believe that they are highly exaggerating what's going on.
Perhaps I missed this in your OP, but has he sent you the address that supposedly has $4k stuck in it? I just find it hard to believe that someone who bothered to buy a hardware wallet and put a non-trivial amount in it could be so careless. I agree with you that his explanation about forks seems odd, maybe he's just a crypto noob.