After the boss is disposed of and Tony is talking to Mel...

Mel actually says "he was hot...over the broad"...indicating that the altercation between Frank and Tony over Elvira is what directly led to the hit.

Never paid attention to Mel's lines in that scene but his words solved the puzzle.

Mel "I told him it made no sense since you were working for us"...Mel ever the practical guy, thought about all the moneh he'd be missing out on once Tony was hit..so he thought the hit was a bad idea..and later.....refused to give Frank any help.