This is a big problem with some sets.
The Targets friend has to be neutralised before she has the chance to tell the target that she has a thing for you, and to call dibs on you.
You really need a faithful wing who can swoop in while you isolate.
Failing that I only have two suggestions:

1) Bang the target's friend. Fuck it, you're getting laid tonight

2) (Very risky, still working on it.) A gag I pull at parties sometimes is to show people 'my new belt'. This involves subtly bringing your balls from your boxers so they are hanging over your belt buckle. You then hang your tshirt over your balls, so to the obsever you are looking normal. When the target's friend flirts or gives obvious IOI's ask her if she would like to see your new belt. When she looks down lift your t-shirt revealing your balls. This is only to be done when you are in a relaxed set which is comfortable with you and that has been finding you entertaining.
Sound crazy? Well I bet negging sounded a bit crazy the first time you heard about it. Let me explain the theory...
If deployed properly the set should laugh, along with the targets friend (so she doesn't seem pissed off and sour). It is also a subtle way of showing your target that you are not interessted in her friend and you are therefore open game. The only problem is that you will probably come off as immature to alot of sets, so DHV is very important before and after the 'new belt' routine is deployed.

This has worked once. I was in a different country and the set was half drunk. But it worked and it needs to be tested again.

Go forth! show thy balls!

Any other routines less likely to end up in a law suit?

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Girls fake orgasms beacause they think men care...

I would neg your target HARD, and possibly compliment the obstacle (to lower your value relative to the obstacle). This situation is much easier to deal with if you have a wing.

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