I had an '03 4.8. He will get home safely and be fine on short, local trips, if flat'ish. But he will quickly feel the need to upgrade. We towed right at 4k with ours. On the rolling hills of I-26 through South Carolina, I would have to play every hill to keep it in the power-band. The good old days. :)
As far as your responsibility...I would have told the buyer what I just stated above as my experience. It would be off my worry, and likely not change the buyers mind, as once excited about it, he/she will be optimistic about their truck. In all reality, his truck is more capable than most any tow vehicle around a few decades ago. I think it's our tolerances that have changed. You know Dezi had her wound up in "The Long Long Trailer". :)