Yes.. he has to be online.. but only when the boss dies.. if you go offline AFTER the boss dies and then log in again you can still get the loot.
So quicly log on you main when your alt is standing by the boss..
You will still see your alt standing there and he is online for around 2mins..
Kill the boss while your alt is standing there.. If he "poof" disapears you where to slow..
Tested this like 5mins ago.. works!