Go to Contacts and select your girlfriend's name. Now go to the "add field" option and select Phonetic First Name. Now add the name how it's supposed to be pronounced.
For instance, my friend Jon spells his without the 'h' and when I tell Siri "text Jon" it would say, "there's no John in your contacts". After I added "John" under the phonetic setting, it now works when I say "text Jon".