Some more possible solutions...
Is your girlfriend using an app that modifies the call functions at all? If so, there could be a conflict between that app and the stock functions of the phone.
The speaker phone might be coming on because the phone is in 'car mode' or thinks it is. (In car mode, the phone defaults to the speaker phone when in a call.) Try a 'soft reset' to clear this: Power off the phone > Remove battery for 30 seconds > Replace battery and power on.
There may be a problem with the phone's USB socket. Check it's clean and clear of dust, etc, and that the pin isn't twisted or bent. A dirty or faulty USB can cause the phone to be in 'car mode' when it shouldn't.
All apps > Dock mode > Menu > Settings > Auto-launch - deselect.
Other than that, there's alway the 'factory reset' option, which will wipe all personal data on the phone.