If the ally is dead you would need a revivify scroll. I don't think we should imbue healing potions with the ability to raise the dead even on a natural 20.
Sorry my mistake, I meant dying so before the three failed death saving throws or someone speeded up the process by using a war hammer. Of course dead people can only be revived using the appropriate high level cleric spells or the scrolls containing them.
I am also not quite sure whether a prone character still gets a reaction...
