Your party member isn’t going to get any additional rewards or loot if she kills the boss while you’re dead. In fact, it’d be more beneficial for her if you were alive, as something might drop for you she wants. If she kills the boss alone, she gets loot for one person. If you kill the boss together, you each get loot for one person, slightly increasing the odds something good will drop for one or both of you. If you’re a good sport and share drops you can’t use, but she can, then you’re encouraging teamwork and fair play.
If your party member is going to make sure you stay dead, it’d be for no other reason beyond causing you grief — like I said, this person won’t benefit at all from you being dead. In that case, this isn’t a person you should continue to group with.
This seems a frivolous complaint to me, and not just because I’m looking forward to playing Hardcore mode.
Here’s a suggestion; if you want boss drops, don’t die! There are (virtually) no death penalties in softcore D3 as it is, and any thumb-mashing tool can beat the game just by zerging things, but now it’s too much to ask that you are present in non-spectral form to pick up your loot?
D3 does not equal D2, but still… if you died in D2 and didn’t get back to the battle in time, you got nothing but whatever cracked sashes the other players didn’t want. So now that D3 has custom drops, you want one even if you’re not there?
Sure, it would be nice if your party waited for you to get back from the checkpoint, but it’s not like they’re standing around playing rock-paper-scissors to kill time; they’re fighting the same boss and its minions that just killed you, they’re probably near death themselves, and the fight just got tougher since your dumb ass bought the farm. So now it’s their fault for not waiting for you? Just… no.