Kudos for keeping your head and being the bigger person thus far.
I would most likely handle the situation by sitting down with the costumer (e-mails can come off so catty sometimes) and explaining that the costumes look great, and everyone is really happy with the work, but unfortunately having one actor who cannot execute the choreography in their current costume just isn't going to help the show run smoothly. Also I'd add that while the slip was a great idea, it seems it wasn't going to work with the choreography either. Offer to have your assistant, wardrobe person, or yourself help acquire and test out a different solution. (I wouldn't continue the blame game on who broke it/actor carelessness..thats dirty politics)
Did you suggest the slip? Or just say that her costume was too revealing for choreo (high kicks, etc)? I know that if you try and tell a designer what to do instead of presenting the problem and asking for their solution, things can get tricky. No one wants to be told by another person on the show how to to their job.
Just presenting the issue in a more personal way (rather than in reports) can go a long way, I've found. The costumer may feel like they're under incredible amounts of stress, and everyone seeing on the report that there's a costume issue might add to it. Try and look at it from their point of view, and when all else fails, remind them how much you appreciate them.
Good luck! Let us know how it goes.