Dude this is a test. Your girlfriend is testing to see whether you love her or you love her hair. The question is which is it?
There are two kinds of these charities. One is a person having their hair cut to actually raise donations for cancer research. The other is getting your haircut to give the money to Locks of Love which claims they make wigs for kids going through chemotherapy. If your girlfriend wants to raise money for cancer I like the idea of making a similar donation to save her hair. If it is Locks of Love maybe what you could do is if they need 10 inches have her cut off of foot and give that and save the rest.
If she really wants to do this as a symbolic thing try to find a happy medium maybe have them cut it up bra strap length. I know it won't be quite as much fun but you can still have a lot of fun with bra strap length hair believe me.