That's a huge request. What, specifically is it for, item by item and cost by cost?
I'd probably say the same thing as seller unless it's major items that should be clearly apparent to seller objectively, as opposed to an overly aggressive try by buyer.
We had that happen at our last closing. Buyer's agent dumped a request at closing for $4k on the table. We told her no. After some tense conversation including blatantly ignoring her and talking directly to the buyer about how her agent was about to kill the deal, we offered $900 or we walk and sale is off. Buyer realized agent was killing her and accepted.
Really have to know the specifics of this situation to know how reasonable, or not, one or both are being.