Not only in the East coast. It's in use (in slight variations, such as cumpà/cumpari) all along the Southern coast and around the entire island.'Mbare' explained on Untranslatable
(n.) Used to refer to a friend in his presence, like "buddy".
"Comu semu, mbare, tutt'a posto?"
"How are you, buddy, you all right?"
Urban Dictionary: mbare
Like dude, mate, bro or cuz. Used in the east coast of sicily. Its also used like intercalary a lot of times