5th dnd – Should the curse of death affect a PC undead in the adventure of the tomb of annihilation?

I will start playing adventure D & D 5th Tomb of annihilation for a group of level 5 PCs that have just completed the Lost Phandelver Mine Adventure Starter Set.

One of the PCs is undead. In his past, he was killed in the Civil War of the Gnomes and resurrected by the Velsharoon himself as a undead gnome with his soul inside his corpse.

Now, I understand that means that he is affected by the curse spell; he should have only 9 HP maximum, and after 9 days this PC would die.

It would be a very stupid and unexpected death, because it is mainly my mistake (as GM) not to warn this player and allow him to use this PC.

Is there a way to invent a solution or modify the Death Curse effect to save it?