Michael Jackson‘s family must be going ballistic over this … Conrad Murray had a delightful lunch with Janet Jackson‘s ex-husband Friday.

The man who killed Michael dined at Crustacean in Bev Hills … with the guy who was once his neighbor.  You see, James DeBarge was jailed in 2011 after being busted on drug and weapons offenses.  Michael’s former brother-in-law and Murray were in the same cell block at L.A. County Jail.

The chick is Nicole Alvarez … Murray’s most recent baby mama.