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This question must be addressed on two levels. From a logical perspective, it seems like the answer would be that she may be turned off if you fuck her friend. From an evolutionary perspective, you should do your best to fuck as many of her friends as you can. Let me explain.
Women are hardwired to seek out the man who has the highest "value." Value is based on a number of factors, but most relevant here is value attributed to a man by virtue of other women. In caveman times, the leader of the tribe was the man who was banging all of the women and passing on his DNA. The women developed heightened sensitivity to determine which man had the highest evolutionary value, in order to ensure survival and replication. The women did not want to fuck men of lower value, because their offspring would be chodes and probably die. This mechanism still operates in the female brain today, whether she wants to admit it or not. From an evolutionary perspective, if her friends like you and want to fuck you, you are a high value male, and thus more attractive.
From a logical perspective, the answer may be different. Other human emotions such as jealousy may interfere. Since she said it would be "weird", but she couldn't seem to explain why it would be "weird", it's likely that she's implying that she would be jealous of her friend. Understand, however, that female psychology is different than male psychology, and although you would be turned off if she banged one of your friends, the same feeling will not necessarily ensue if you bang one of her friends.
For these reasons, I would advise you to bang her friend, but make it seem like it just "happened", and it wasn't something that you planned. The heightened value within the social circle will probably outweigh the potential detriment of jealous feelings.
While this is very detailed... the answer has fuck all to do with DNA.
You fucked her friend. Yes, you're screwed... Unless you can swing a threesome.