I got her the 32 GB iPad 4 3G + Wifi So is the 3G (as in, the sim card slot) only for a data sim card that I have to get her a special data plan only sim card on or can she put her normal number which also has a data plan and use the iPad? the iPad can not be used as a phone right? it's just for the sake of data right?
That's correct. While you might theoretically be able to use an existing SIM from a smartphone with a data plan, obviously you would lose phone functionality because the SIM can only be in one device at a time. There are some VoIP apps available if you want to make phone calls, would look pretty strange holding an iPad to your face though.