I've never heard of anyone getting that riled over that. I'm getting married about less than a week and the parents of the groom never sent in their RSVP... And I'm totally cool with it.
1. I know they're coming. They told me. They know I know...There is no doubt in my mind.
2. However, I can understand her point of view at least a tiny bit. Being the bride means nearly every decision and project ends up at least partially your responsibility. So, stress is just expected. Maybe brides overreact to simple issues just because of the stress they're feeling. Also, emotions and hormones are possibly running crazy too. If she is working out feminine issues for the wedding night, she could be more emotional than usual as well.
Don't take me wrong. I think she needs to take a deep breath and realize that getting really upset because of something this insignificant is just not worth the energy when there are so many others things that need her attention. Maybe her future hubby could help her realize that she's going a bit overboard and needs to choose her battles. But, it wouldn't hurt to apologize for not making it abundantly clear - just make her feel better. If she feels better, everyone else will be happier in the long run.
Hope this isn't too long and hope it helps.
When all is said and done, they'll be married and happy. That's all that really matters.