If the honoree decided to throw a "happy being single" party, and it was her own idea, I think that would be fine. As long as there's no mention of gifts, and no one else is expected to foot the bill, it sounds like a nice idea for a great party. I can also see the parents thinking "well, she's not getting married, so may as well give her the money we would have spent on a wedding and have a fantastic party" (although I'm not at all in love with the idea of "only giving money to the kids when they get married" - but that's another story).
That being said, I think it would be terrible if the mother decided this and went ahead with the party. Even if the daughter is a confirmed bachelorette, it should really be her decision if she wants to celebrate that fact.
Of course, registering for gifts as a way to get paid back is beyond the pale (wasn't it in an episode of "Sex and the City"?). Then again, I think it's a commentary on exchanging gifts in the U.S. today - people celebrate these milestones with big parties (fine) and mile-long registry lists (not fine).