Bottom line, friends aren't therapists, and offering too much intervention is bad for the person being pushed to offer it. It is also often bad for the person with mental health issues.
For example, a friend not having proper boundaries around phone calls/visits/how much they can do will burn out pretty quickly. The person with mental health challenges will come to expect them to be available, and then they simply will no longer be able to manage that for any number of reasons. And out of desperation and to protect themselves the
It is always kinder to simply say no to excessive demands from people with mental health issues, right from the beginning, as they are unsustainable. But most people only learn that several months in to being pushed into the role of support person. That's when people vanish, as it is just far too much and they cannot deal with it, no longer want to, do not want to be shamed or guilted for just being an ordinary human dealing with their own stuff.
Even worse can be those who really do make their whole life into supporting their mentally unwell friend, those people often have their own personality chalenges.
Therapists and care workers have supports in place, psychologists etc have to debrief with other psychologists because the weight of other people's struggles is too difficult even for trained professionals without other professionals to support them.
Friends aren't support workers, carers or therapists and they have a right to draw their own line in the sand. It's always safer, fairer and better to leave mental health support to those with training in the field.