and Guest dilemma **Updated**

The entire 'and Guest' thing has been giving me headaches lately, but this one takes the cake...

I invited a girl I taught with last year to the wedding. We were super close, but since she changed schools, we really don't speak much anymore. I did invite her out of guilt since every time we do talk all she wants to hear about it the wedding, etc. I addressed the invite to only her without a guest because she will know a lot of people there anyway.

Well today, I get her reply and she added a guest for herself! The nerve of some people!

My question for you ladies is how do I tell her that she can't bring a date, while still being tactful and without hurting her feelings?

Thanks

*************UPDATE*************************************

So for all of you ladies who offered tactful ways to approach my friend, thank you. But I will say, all was lost on her. She didn't get why she couldn't have a guest, even after I said it was about the $$$. She said she figured she would get back together with her boyfriend by the wedding and she figured it wouldn't be an issue.

In closing she said that if everyone else from my school was invited with a guest, she didn't think it was fair that she was not...I HATE PEOPLE!! I should have never invited her