Welcome to Android Central! Also try Samsung's Find My Mobile.
If your girlfriend didn't have a screenlock set, use Android Device Manager to remotely lock and/or wipe the phone: https://support.google.com/accounts/answer/6160500?hl=en. Also, tell her to change her Google password now -- if the person who has the phone can access it without a screenlock, that means they can access her sensitive Google data unless she changes her password.
Your phone carrier might also have installed security software. For example Verizon has security / anti virus software it asks to install that can find erase etc a phone.