As a referee, what should I do if I think the other reviewer of a paper is not objective?

My perspective as an Elsevier editor: This happens sometimes and is easy to see through. An outright "accept" on a first submission is so rare as to raise eyebrows in any case (at least, in my field). The editor has probably noticed the lack of thorough review by this referee and will give the review the appropriate weight (i.e. none). That said, I would not see any harm in emailing the editor if you are concerned, so long as you keep it polite and to the point.

Transparency is key. If you suspect some kind of fraud, be open with the editor, and send an email to prove you acted proactively. If your suspicion is strong and you do not act clearly, you risk later appearing as an accomplice.

Probably the other reviewer is a friend and is also superficial (because he is riskying highly if discovered). This is a typical proof that the peer review system is, somewhat quoting Churchill's definition of democracy, the worst system to disseminate research findings but all the others which have been tried before.

Ultimately this is up to the action editor. The reviewer you speak of hasn't hidden his/her obvious affiliation with the authors, so hopefully that will be noticed. A good action editor has sole discretion, with the peer reviews as supporting evidence. A terse 'accept' review may carry no weight, but tough to say. Most likely, if you made great points, they will be passed along.