My understanding of Steam Guard is that it just requires a more thorough verification if you try to log in on a different computer, not that it actually prevents you from logging in on different computers altogether.
Yeah the process of steam guard makes it s a bit more fiddly than the current case of just needing to know each others log-in. And it'd be even more fiddly if she was the first to log in to a PC with the account (though that was going to be pretty unlikely)
Now you need to know each others log-in, and then contact each other to get the code off them.