I went through something like this with my ex.

One day, she was fine.

The next day, it was all about The Beatles conspiracy theories which never seem to end. Ever.

I stuck with her for two years before finally, reluctantly, leaving. We divorced a few years later, by which time her psychotic break had finished.

It was truly awful. I didn't understand it at the time, and was angry then guilty. I should have dragged her to a hospital for examination, but I didn't.

Get your friend some professional help.