No matter what, after the duration and the spell ends the monkey goes back to its home plane, death of the monkey immediately ends the spell, so yes the monkey returns to its home plane.
The monkey family is all sitting in a tree, eating, when the eldest son suddenly disappears. After freaking out for a minute or two, the monkeys see a flash of light from the tree next to theirs. Overjoyed, they rush over to find that the eldest son had come back dead. So sad!