Ive been in a similar situation and it took me about 10 years of work to get mentally healthy, and I am now 36 and been working in the mental health sphere (both young people primarily with neurodivergence and elder people with a combination of physical and mental issues) for a few years where my experiences are a great asset.

Your person might have it super rough, but each time he/she admits herself rather than being forced is a great step to mental health and it should be focused on with positive light. It is never too late to get better, and a misstep is only a failure if the person gives up, otherwise it's just a hiccup and those happens.

Remember to also take care of yourself and make sure to have healthy boundaries. A close person are never a professional mental health worker with responsibilities and need to remember that so they don't drown. A person need to be in a good position to be able to help, or it will just devolve into another person feeling unwell.

If you are interested in helping your person or anyone else with mental health, I recommend practicing enforcing personal boundaries as much as possible and when you (you as in anyone reading) feel secure and safe with your boundaries take a course. There might be MHFA courses or similar where you live.

MHFA:

https://www.firstaid.se/en/info/mental-health-first-aid-mhfa/