That sounds like a very difficult experience, I’m sorry for your friend.

At risk of overstepping, I wonder if the underlying issue might be better viewed as needing to rectify and update previous ways of being, self-models, and goals with the new circumstances. As well as fully grieving this loss, coming to terms with how things are, and learning to imagine all of the new possibilities that can still be realized.

Sure, but isn't that exactly what we would be trying to treat? I would like to believe in hallucinogenic therapy, but if it can cause an underlying problem to expand to the point where it causes a negative change in behavior, that's not good. It would certainly depend on the risk, but having a risk of triggering a mental illness the person was previously coping with is not a good thing.

Finding a way to deal with your problems is the best solution. In lieu of that, I would argue coping with them without handling them is the second best, with neither handling them not coping them being a distant last place.

I would argue that in very specific cases being in ignorance/denial is better than wholly accepting your circumstances.