Simplest first, good old counterspell
Counterspell is the first thing that comes to mind and just needs a reaction, a 3rd level spell slot, and a PC to make a 19 DC ability check. Not a guarantee, but gives you a good chance of preventing the spell if it comes down to the wire.
Certain spells and abilities can help buff the party's hit points
Since the spell fails if the target is over 100 hit points, buffing hit points and max hit points can help here a lot. Aid immediately comes to mind to raise your hit point maximum starting at 5 at 2nd level and scaling 5 per level. If you can afford a 5th level spell slot, that's 20 hit points to everyone's max hit points are extended by. In combination with bonus hit points, which can be given with many sources two that come to mind are the Inspiring Leader feat and Heroism, you can easily have enough extra hit points to delay anyone getting to a point where they can be affected.
As a fail safe, bring revivify
If all suggestions fail, there are still ways to bring back a PC. Revivify for a cost can bring them back to life with 1 hit point to at least help minimize the consequences of power word kill.
There are other things that can pair with the above to help like using things like Bardic Inspiration, guidance, enhance ability, or even a divination wizard's portent can make the counterspell pretty much guaranteed. Bringing in resistances/healing along with any resistances like a raging bear barbarian or other effects that give resistances to damage types your hit points will last longer. If revivify itself is not available, it's possible to recreate it with wish.
Nothing above is foolproof, but it should give you enough combinations of tools to get over power word kill. I'd honestly be a bit more worried about the Lich up casting something like cloud kill to 9th level. 9d8 per turn damage even with a save may not have the same oomph as an instant kill, but it being an area of effect a party of 4 can easily be dealt over 100 damage on just the initial casting.