It would help if you could post details of the OS versions of each Blackberry, which carrier each of you is on, and which PC OS you're using.
I set my 8310 up recently following the instructions here:
http://www.blackberryfaq.com/index.p...Berry_As_Modem
The only problem that I had was an inability to follow instructions; I initially added it as a "Standard 56k Modem" rather than a "Standard Modem" which didn't work.
If you haven't done that, and the Blackberry modem replies when you query it, I'm guessing that your girlfriend's SIM doesn't allow access to the whole Internet, or you haven't set up the APN correctly. Once you post details of what carrier she's on I'm sure that someone will chime in with the APN information. You could always test it using your BB on her PC of course (making sure that you don't inadvertently change her email accounts to use your BB) or even, if the devices aren't locked to different carriers, your SIM in her BB.