I would be more specific with what the issues are rather than referring them to a Youtube channel which they probably won't watch. Not everyone has the time or inclination to watch a lot of Youtube. Also, many adults associate Youtube with young people and children, and many pet Youtubers are also relatively young, so if your argument is based on Youtube, it may come across not very trustworthy (especially if you yourself happen to be under 18). Youtube and social media in general has a well-deserved reputation for not being a good source of information.
If you are able to point something out about the rabbit that supports what you're saying, that can really help. So check the rabbit's feet first. If you see any signs of soreness or any cuts, you can point them out to the owner and suggest that putting a solid floor in the cage instead the wire will help. People will often notice and appreciate your thoroughness and attention to detail, as it proves that you have been caring for the rabbit and looking out for its welfare, not just putting food in every day.
Personally, about the hay, I would be tempted to say something along the lines of "does your rabbit eat hay? I couldn't find any with the supplies you left and rabbits need to eat hay as the majority of their diet". If they come back and say "he doesn't eat hay" I might say "rabbits can get GI stasis if they don't eat enough hay, which is really serious". If he's just eating pellets he might be overweight (or will become overweight) and because excess weight makes it harder for them to clean themselves, he might have a sticky or dirty bottom, which attracts flies and increases the chance of fly-strike (even more so as he's outside, and summer has just finished) which is a horrible and often fatal disease. That's another thing you could point out to them. You could also link to a reliable-looking non-social-media website such as this, to back up your point.
I know it's hard to be direct (you don't have to be super direct, as others have said, don't just say "your care is bad", "there are issues with your care" etc, in fact just avoid using a negative adjective and "you" or "your" in the same sentence) but many people are able to recognise when someone is well-meaning and appreciate it. It's often easier to do it over email (if you're sending regular updates and photos, you could do it then) rather than in person.
Now there's always the possibility they will pretend to listen to your advice and then do nothing, or genuinely intend to do something, but then forget. But that isn't your fault as long as you've tried.