I can identify with the guilt. In my case with my friend, their behavior lead to me distancing myself from them, not knowing what was going on, and I was his main source of socializing. Same thing happened with several of his other friends. The isolation definitely was something that amplified the progression of his illness to detachment with reality. Didn't help that the people he sought friendship from in lieu of us were scammers who fed into his delusions to take what little he had even when he became homeless.

If I could go back in time, I would do things differently, but at the same time I can't blame myself for not understanding what was happening and doing what was, at the time, the healthiest thing for myself.

Sorry to hear about your friend.