I read that Cassel introduced John to Yoko, then Johny Cash to John, and Yoko to Bukowski and Belushi (who were a couple at the time, living in a cute West Hollywood Craftsman, according to Sounes). So this guy was busy.
Then, according to Sounes' reading of Cassel's stolen mail, Yoko came to West Hollywood to do an in-store record signing and Belushi introduced her to freebasing while Bukowski took pictures (you may have seen some of the pictures in May Pang's book). In one famous picture Cash is massaging Bukowski's shoulders while Yoko administers a golden shower to a prone, naked Belushi. Cassel is nowhere to be seen. He was an instigator!
Johnny Cash later said that while those days are some of his fondest memories, he would still clobber Cassel if he ever saw him again, because Cassel had taken polaroids of June Carter Cash on the toilet and sold them to STERN magazine. I don't know if that last part is true, but Sounes threatened me with a lawsuit for mentioning it once at a party.