Sorry, practical point - you mentioned that the poor chap has also been diagnosed with coeliac? There's definitely a proportion of the older bedwetting young people who are subconsciously masking constant gut signals, and that makes it harder for the weeing signals to get through. Treating his coeliac may actually help the wetting, but if he has any degree of constipation, please please get that treated. Having pooing perfect, rather than okay, really makes the difference in some of these kids, and gives a window of opportunity for getting real improvement.
I would actually agree that using a decent quality pullup/nappy can delay progress. But there's a really big but. If they're ready to improve, nappies can delay things. If their body isn't ready, being wet every night is miserable and can get them sore, so using nappies for term times is fine, as long as you give it a try during holidays. Some people find wearing underwear under the nappies is a useful halfway stage - that way they do feel wet, but the poor kid isn't lying in a cold soggy bed.
Oh, and by now I'm sure you've done most lifestyle stuff the best you possibly can, but wanted to make sure he was drinking lots (at least 2 litres) earlier in the day, and that you'd banned fizzy drinks except as occasional treats.
Oh, and your friend's reaction was not reasonable.