Conversation so far:
How did Dennis die?
he fatally drowned in Marina del Rey.
When did he die?
December 28, 1983.
Was he with the band when he died?
Dennis was given an ultimatum after his last performance in November 1983 to check into rehab for his alcohol problems or be banned from performing live with them.

Question: Did he check into rehab?
Self-contained rewrite: Did Dennis check into rehab for his alcohol problems?