回复：请教美国职场的社交经验

I assume the guy from the other department is the lead of the project. In that case, just let the guy run the show and you play along. A bit of cheer-leading would certainly be helpful to you to build a stronger relationship with the guy. But if you have any concern about his aggressiveness toward the client, then don't respond directly to the emails he copied you. Instead, you express In person your support to him and your appreciation for his dealing with the tough client. At the same time, you need to discuss your concern with the boss in your department. He will provide you with further guidance. At least, you can keep yourself out of the mess...