Mower doesn't like new customer

I picked up a good 4500 craftsman the other day. A farely newer model. Not one with a PTO. It had a 20hp OHV Briggs on it. Just a few minor things wrong with it. Valve adjustment, new fuel lines and filter. It ran great. After the valve adjustment, it started and ran great.
I sold it to a guy. He came and pick it up. It started fine when he got here. Said it started fine when he unloaded it. Later that afternoon, he called and said it wouldn't do anything, when he turned the key. So, the next morning, I drove all the way out there, (20 miles away). Got on it, and it fired right up.
I told him he'd just have to get used to certain things. I sort of figured that maybe he didn't have the clutch pressed all the way or something. Anyways, the next morning he called me and said it would turn over, but it would drag the battery down too fast for it to start.
Even though he knows there's no warranty on use mowers, he still wants me to drive all the way out there and fix it. And I'm not sure what it could be, because I've replaced the battery. Checked all the safety switches, adjusted the valves. And it worked great for me, every time.

Mower doesn't like new customer