Log out other user w/o first switching to that user

Using the Terminal, you can kill her loginwindow process and any programs she has open will be closed... but this will cause her to lose any unsaved work she has! In fact, this is the very reason you need to log in as her to log out: When you log in as her, all her programs again have access to the GUI, so they can prompt you to confirm closure, save changes or take other action before logging out.

If you're sure that she only has things like a web browser, iTunes, etc open, not Pages, Word, Photoshop, or anything else with documents, then you could try from the terminal:

kill `ps awwwwux | grep her_short_username | grep loginwindow | grep -v grep | awk "{ print \$2 }"`

On OSX 10.10.4: (slight edit from last comment):

export pn=`ps awwwwux | awk '/her_short_username/ && /loginwind[o]w/ { print $2 }'`
sudo kill -9 $pn

You can use the Activity Monitor to log another user out by killing their login process:

1. Run Activity Monitor
2. In the filter at the top-right, type loginwindow
3. Click the row with the user's name in the User column
4. Click Quit Process, then Force Quit, and type your password.

This is just a nicer(?) GUI version of Josh's answer.

Before step 2, you can review the other user's running processes to see if they are running any apps that might lose data on logout.