Name changed, as this will potentially make me identifiable IRL.
So, as long as I can remember my dad has always been slightly New Agey. I'm a complete sceptic - partially due to some of his antics during my childhood - but have always accepted his ways.
Some recent developments, however, have me really worried; he's basically gone from slightly eccentric to what is IMHO borderline insane:
Yesterday, I met one of his work colleagues on the train. We chatted a while and that person told me how sorry they were that my dad was leaving the company - and a job that he has always said was his life's dream. I was even more surprised to learn that he had also apparently given up on his band, another long-standing passion. As this was all news to me, so I naturally called my father to find out what's going on.
As per my dad, he's giving up the band because he has recently started channelling music from some extraterrestrial guide, who is apparenty some 20000 years old and is dictating him messages for all of human kind in the form of guitar scores. My father's band (understandably, IMO) did not want to play the xtraterrestrial music instead of their usual repertoire and my dad hence feels that he needs to go his own way musically. 
He also feels that he is being called to be a messenger for the extraterrestrials, which is why he won't be able to hold down a full-time job. 
Now, I don't doubt for even one second that this is what he perceives. As an outsider, however, I'm wondering if he's having some kind of a breakdown and this really worries me. His complete disconnect from reality does not sound healthy to me.
I'm obviously also really worried about him giving up a job on a whim - my dad is in his fifties and his industry is not doing well at the moment, so it is doubtful if he'll get another job in his field. He also seems to be disconnecting from people like his band mates - his best friends for 10+ years. Frankly, it sounds like some kind of a mental health issue to me at this stage (he has a history) and I really feel I should somehow interbene.
What do I do in this situation? Can I do anything at all? Should I, or is this basically his own thing?