It's sad that they let this opportunity slide by...

I agree with 5LM, I wouldn't mention the other kids successes in comparison, but I think it would be ok to slip it in casually - like "reminding" the other kids at pick up to tell you if they have to go, or praising them to their parents when dcb is picked up.

I think you've fine your part. I'm in a similar spot with a dcb (almost 4!!!). His parents have never committed even though they've deceived me into intense training twice, while they sloughed it off at home. I recently gave up. Through in the towel (through their protests, hilariously) when dcb started peeling on his clothing during potty time instead of on the potty. It was clearly a behavioural issue.

I'd tell dcd that you won't fight with dcb over the potty. If he isn't willing to go on his own, it won't happen. I told my dc parents the same thing. If the child is showing me they don't want to go, they don't go. There's only so much praise and rewarding you can do, at some point the parents need to take responsibility or it isn't going to work.

Personally though, if dcb is willing to go, I'd let him get back into routine (regardless of what the parents are doing) - I'd rather deal with it in the potty than in the diaper, personally. And I'd hate to be the one to hold him back.

With enough positive modelling, he might get back into it eventually.