if she's using a Creative card, tell her to change her pin line from "What U Hear" (yes, that's actually what it says) to Microphone. on other cards there's usually a wave out pin line, but it's usually called like stereo mix like david said or something similar.
to change the pin line, like david said open the volume control panel, but don't mute the channel. instead, go to options > properties. change 'adjust volume for' from playback to recording, then make sure microphone is checked in the box below. hit ok, then on the main volume control panel screen check the 'select' mark underneath the microphone channel.
this is all assuming she's using primarily WDM (ie: not ASIO) drivers. if she has an ASIO card, those vary from manufacturer to manufacturer. in some cases she may in fact be shit out of luck considering ventrilo relies solely on WDM channels, of which on most ASIO cards there will only be one consisting of all sound.
i will shit myself the day ventrilo (and skype) adds ASIO capability.