I remember something about CON damage not killing you unless you hit 0. The argument went like this?
At each level, you get D(something) + your CON modifier.
First level Bob the wizard (with a 14 CON) = 4 + 2 = 6 HP
If you loose CON, that modifier goes down (maybe negative) and this total number of HP gained at each level goes down.
Poisoned Bob (with a 14 CON, but poison lowers it to 9) = 4 ?1 + 3
The difference between 3 and 6 is 3, so he takes 3 HP from the poison.
But, you always get at least one hit point. So, if Bob ends up with a CON of 1?
Really poisoned Bob (con now 1) = 4 ? 5 = -1 BUT you always get one per level so 1
The difference between ?1 and 6 is 7, so Bob should take 7 HP, but in this case the ?always at least one per level? rule saved him, he is at one hit point. Note that he would be at 2 HP if he were bob the second level wizard.
In short, CON damage can not put you below your level in HP unless your CON gets to 0, in which case you die.
I think this rule will save the druid. He will loose his 4 CON if the DM rules that your bear form can?t wear the amulet. That SHOULD cost him 22 Hp, but since that would put him below 11 HP (one per level) he?s ok.