You work with an older child the same way you work with anyone of age. Respect is very important. People hate most being told what to do. They want to decide for themselves. Sometimes we get in this situation with parents, and they fight us, for they hate being treated like a child, and yet they are no longer competent.
If an adult child is not longer competent, and they make decisions that will hurt them, or decisions that are unwise, sometimes all you can do is stand aside and watch them go down. You can offer help, and keep offering it, but you can’t make them take it.
I have seen children in their thirties, suffering from migraines and depression, and their parents hospitalize them when they are really sick, and care for them at home when they are well enough. It’s not how you want to see a child, but if your child will take the help, you can give it.
So much of this depends, I think, on the past, and what your relationship was like when they were growing up. If it was a contentious relationship, with mistrust on both sides, it will be hard to turn that around. If you had a more trusting relationship, a more loving relationship, then my guess is that that will make it easier to help them as adults.
For some of us, the time after college is when we break away. I know with my parents I never felt particularly loved or appreciated. Throughout my twenties, I was struggling to do something they would be proud of. Eventually, I gave up on that hope, and learned to do things for my own goals. When I got married (I was in my thirties), our relationship settled into something—if not loving, at least occassionally warm.
I say this to suggest that there could be a time when things are more difficult, but you may eventually grow closer again. It’s so complex, though, and so many factors are involved, that without much more detail, it is really impossible to give you any meaningful advice.