You need to talk to her about it. If she says no, do not pressure her to. But you can cause a situation that allows her to let you. Next time you two are going to have sex (assuming you two already do), accidentally forget to bring a condom. Do not tell her you do not have it until you have her already ready to fool around and have her in the mood. Then you can say "we can always try anal."
Unfortunately this is not something that has a definite solution to as anal is very painful for a girl. You will just need to get her to trust you that if she is in pain you will take will not try to force it in or continue going while he is hurting.
Possibly start with anal fingering to get her used to the feel of it.