Rentals are one-time only downloads, so if she's downloaded it on her own computer then you won't be able to download it on the iMac.
From http://support.apple.com/kb/HT1657 :
If you download a rented movie on your computer: You can transfer it to a device such as your Apple TV (1st generation), iPhone, iPad, or iPod if it’s a standard-definition film (movies in HD can only be watched on your computer, iPad, iPhone 4 or later, iPod touch (4th generation or later), or Apple TV). Once you move the movie from your computer to a device, the movie will disappear from your computer's iTunes library. You can move the movie between devices as many times as you wish during the rental period, but the movie can only exist on one device at a time.
So I'm not sure if you can move it between computers via home sharing, or whether you would need to move it to a device and then from that device to the iMac.