A lot of domain owners think their domain names are worth much more than they are in reality.
A domain name is worth what someone is willing to pay for it.
You can try to get the buyer to pay more. Just tell him that the owner thinks it is worth more and ask him if he has a better offer.
Personally, I think your advice is correct. The owner should consider taking the offer, but watch out for scams. If the buyer wants the owner to pay for a domain appraisal, just walk away from the deal. It is likely a scam.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin