This solution will require you to have exceptional control of your gastrointestinal processes, but if you do........
Agree to do what she wants, but make sure she takes her cloths off first. Once she is completely naked, begin vomiting uncontrollably. I don't mean a little gag, here, I'm talking lose breakfast, lunch, and dinner and hit her with it, if at all possible.
She will never again be interested in you sexually, she will probably run to her boyfriend needing to feel desirable again, thus drawing them closer together, and she certainly won't tell anyone about you being so revulsed at sight of her naked body that you vomited.
Actually that would probably be easier than what you really need to do which is, tell him. You may lose a friend(I hope not), but it's better to lose him through truth and truly being his friend, than allowing her to come between you, because she's going to say it was your fault either way.
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"The Martyr" (excerpt)
There is sobbing of the strong,
And a pall upon the land;
But the People in their weeping
Bare the iron hand:
Beware the People weeping
When they bare the iron hand.
--Herman Melville (written after the assasination of President Abraham Lincoln)
[This message has been edited by Ronn_Bman (edited 10-21-2001).]