。。
Cut it out. He's our friend, isn't he.
Literal
Stop. That-guy [topic-は] our friends [copula-だろ].
やめろよ uses the rough imperative of やめる + emphatic よ. あいつ 'that guy' is a rough, casual pronoun. 俺たち 'we/us' uses the masculine 俺 with plural たち. だろ is the plain-form of でしょう seeking agreement, with a confrontational edge here — 'he's our friend, right? so stop.'