He hung out with a lot of famous folks, including the Beatles Three Dog Night and the Mamas and Papas. Sadly, Cass Elliott stayed at his apartment in London when she was in the city for a concert at the Palladium. She died at that apartment of heart failure in July of 1974.
Four years later, in early September of 1978, Keith Moon, drummer of The Who stayed at the same apartment when he was visiting London. After dining with his girlfriend as well as Paul and Linda McCarthy, he returned to that apartment. He died the next day of an overdose of a sedative designed to help with alcohol withdrawal.
What a heritage to have for your apartment, right?
A number of Mr. Nilsson’s friends passed away at a relatively young age. Here’s an article for further reading if you’re interested.