What happens to a player who has 1 life and a Phoenix potion, and another player casts Finger of Death on him before he engages him?

Weslocke said:

Phoenix Potion

You may discard this potion when you lose your last life to immediately heal all of your lives.

I don't understand what's the problem. It's because you lose two lives? You can't get below 0 with lives and Spells must be resolved in a single timespan.

It's not possible to lose the first life, recover all lives and lose the second life.

thank you Warlock.