Actually, it sounds to me like FNC might have just dodged a bullet there. If she is a) immature enough not to be willing to apologize for hurting her good friend and b) so uncommitted to dancing that she quit because one partnership ended, then he may be better off with another partner.
I agree that FNC didn't do anything wrong by asking (unless he was close friends with them both and knew in detail what their commitment to the partnership was). In any relationship, there will be outside challenges, everything from schedule difficulties to someone making a pass at a married person. It's up the the members of the relationship to enforce its boundaries and say "no" to things that threaten the relationship.
I do have some sympathy for the lady. How could you pass up a try-out with an FNC? And I can understand not wanting to tell your current partner if you weren't sure the new one would work out, because of the possible emotional complications of just having the try-out. It's sort of like, you don't tell your boss you're going on another interview. You tell him when you get the other job. And maybe she read him right--he did respond pretty dramatically, by quitting dancing altogether. It's easy to say in retrospect he would have been happy for her, but how would he really have felt over time dancing with someone he knew had tried to dump him?
That said, once she made a choice on how she was going to handle it, she had a ethical obligation to make that choice work. Either:
a) be open from the beginning
b) keep the first tryout a secret--really a secret. That means, not being seen at a comp in ballroom hold with FNC, having the tryout somewhere they wouldn't be seen by other ballroomers, and, once she had determined that she wasn't going to go with the new partnership, keeping her mouth shut for eternity.
As far as I'm concerned, it's the half-a$$ed way she handled it that caused all the trouble. And she certainly should have apologized for hurting her partner. Even if she felt it was OK to do the try-out, even if she felt that she was perhaps protecting the partnership by not telling him beforehand about something that was just a possibility, she should be sorry that she hurt him.
