Lets say my friend attacks with Chief of the Edge and two other Warrior creatures, and I declare three blockers, one on each creature. If Chief of the Edge dies due to combat damage from blockers, when do the other Warriors lose the +1/+0? Is it instantly and now his 2 other creatures are normal again?
All combat damage happens simultaneously, and creatures who have taken lethal damage will die almost immediately after damage is assigned (before the next time any player could do anything else; when state-based actions are checked). This means that the warrior creatures will still do the 1 extra damage during the combat which causes Chief of the Edge to die, and will lose the +1/+0 immediately after that.
You're asking "when does the Chief die" but honestly, you need to tell us that, as ironic as that sounds.
If there is "nothing fancy" and there's just his attackers and your blockers, then all his attackers will all get the +1/+0 bonus.
If you kill the Chief with First Strike damage, or if you kill him with an instant or an ability on your Declare Blockers step, then by the time his other warriors do damage the Chief isn't around anymore, so they wouldn't get the bonus.
So really, it all depends on which step the Chief dies on.
The ability on Chief of the Edge is a static ability. It only applies while he is on the battlefield. The instant that Chief dies, the effect created by his ability ceases to exist.
112.3d [...] Static abilities create continuous effects which are active while the permanent with the ability is on the battlefield [...]
If Chief of the Edge takes lethal damage, then the next time state-based actions are checked, he will die, and all other warriors will instantly lose the +1/+0 granted by his ability.