If I were your parents and was financially able to facilitate this (you say they are financially well off), I would present nephew with three options:
First: He proactively takes a full time job.
Second: He returns to education, financially aided by them but he also keeps a part time job to contribute. Any education - academic study, vocational, apprenticeship. It doesn’t need to be a career for life choice, just something that leads to a better job.
Third: He takes some time out to travel and do some volunteer work, financially supported by them for a fixed period of time, after which he fully supports himself.
All three of these options get him out of the rut and get him taking a next step. It’s very sad that he doesn’t seem to have had the parental guidance to set him on the right path, but equally, at age 23, he needs to take ownership of his life and do something - anything - to further his life.
He’s not going to overcome depression by having no purpose and staying at home. Don’t underestimate my third option, travel really gives life experience and resilience. I would make it clear that what is not an option is continuing part time work with no purpose or intention to progress in life, indefinitely subsidised by them.