How about calling it “he got a bit of work experience for free”? He can at least talk about having experience of waiting tables at his next interview (if he didn’t have it before).
Funnily I asked Dave at the local if I could do a shift for him for free before I went to university, in return for him showing me how to serve different drinks properly, change the barrels etc… The idea being it would make it easier for me to get a part time job while I was away. He did, I had a great time and a few beers thrown in too. Never got a bar job at uni though, ended up working in a shop, which I’d already got loads of experience at doing before I went to uni.