It depends which phone she has. The iPhone 5 takes a "nano-SIM" card, which is a different size than the standard SIM. You can apparently cut it to fit with a special tool (or, I guess, scissors, if you are very skilled), but I don't know if I'd try this. I went to some trouble to buy a SIM card for my phone that was nano-sized.
Originally Posted by jeffmac
I have actually done it a couple of times with good scissors but most phone stores have cutters and you can then keep the piece around it if you need to put it back in a MicroSIM sized phone.