Yes there is a chance of pregnancy.
She can either get the plan b pill at a sexual health clinic/ her doctors, or it can be bought at boots after a quick chat with a pharmacist. It's best to be taken within 5 days of the unprotected sex.
I also advise you and your girlfriend have a serious talk after this as to whether your current birth control methods are good enough, as clearly in the event of pregnancy you two have divided opinions. That's not a topic you want to ignore until it's too late.