- The Cock-Block.
Your friend is trying to hook up or otherwise get especially friendly with somebody, but you monopolize the "target's" attention in some way to distract them from your friend's intent. Obviously, this outcome is "poor", unless you're some kind of dickhead that doesn't want your friend departing early with the target. Quite selfish, to try and keep them out with you. Of course, there are all sorts of minor rules variants here, that are rather crass: e.g if you're both interested in the target, who steps forward, who holds back? It's simply best to avoid this scenario because it never turns out well.
- The Wingman.
Oh yah. We all know this one... the friend who props up the other and makes them ever more desirable. Talk up their strengths, ensure that the person-of-interest gets excited to know more about your friend. This is the ideal outcome, especially if they make some kind of lasting connection.
- The Bus-Tosser.
This isn't nearly as bad as the Cock-Block, but your friend isn't going to be all that happy with you. At least for a short while. This is where you think your friend is interested in somebody, so you move into Wingman mode. Provide lots of opportunity for the two to talk and hang out, provide some good commentary, etc. Like any good Wingman would do. But afterwards, you find out your friend was not interested. At all. This is the "thanks for throwing me under the bus" maneuver, putting your friend into harms way. Especially if the purported target is interested and giving undue attention to your friend. ... Thankfully, in the long run, this provides lots of laughable material for how you sucked as a myopic Wingman.
I think the best answer all around is to simply go out and have a great time with your friend. Anything that will involve a possible third person can fall into a poor outcome, or simply distract from an awesome evening with a friend.