Can I download my friend's Steam Family Sharing games while he plays?
When my friend is playing a game on Steam, can I download his games?
Without starting the game of course.
From my own experience, yes you can. As long as the person doesn't actually launch any games they can download all they want. If you want some more info regarding what exactly else you can and cannot do. I recommend checking out the following links
https://support.steampowered.com/kb_article.php?ref=3784-QLBM-5731
http://store.steampowered.com/promotion/familysharing