PC of the living dead

So, I'm working on a new DnD campaign. It's a mid-level evil campaign where the characters are starting by breaking out from prison. That's not quite so important, but one of the characters applying to join the game has the ultimate goal of becoming a lich, which is a great goal for an evil campaign, right?

But what about the other characters? Liches are strong creatures, and when one becomes a lich, there's essentially a four level increase in power all at once. The other players alongside this character have to either get a similar increase in power or deal with an inferiority complex for a few levels. Both have some serious issues.

Becoming a Lich takes some time in-game, so let's say that the character goes off for one adventure to do this, while the other characters adventure. What does the player for that character do? Does he go away while the other characters gain *four* levels? That could be months! So, that's not really a plausable option. Does he play with nerfed powers? That's probably a good idea, although it won't be as fun for the player who now has to wait through even more time in order to complete their transformation. Why spend so much money to have to wait so long to actually get the bang for your buck.

So, we give the player all their powers after, say, one missed session. Now, the other players are four levels behind, give or take. Everything everyone does is incomparable to what the lich can do. He outcasts the casters, he soaks up more damage than the meatshield, he is more effective sneaking around than the rogue. So the other players need to gain power to match. Do they gain four levels to catch up? Then the lich suddenly feels gipped - he just sold his soul to gain power, and now everyone else auto-magically catches up to him for free. Same issue with items, either the Lich has to get a share of some way-overpowered treasure (making him even more powerful), or he'll be gipped. You could potentially squeeze it in while he's off becoming a lich but that'll still be taking from him one of the best bits of becoming the lich in the first place - power.

The best solutions I can come up with are these: Either the player has to spend levels as he approaches lichdom and slowly gain the power it would bestow. It requires building a several-step lich class, but that's a small amount of work for the pain you would otherwise be inflicting upon yourself. The other option is that lichdom requires a sacrifice of several class levels to do so. The player would have to take the option: lich power, or class power. It's not a bad option but it cuts down a lot of things the player's worked for for a good while.

Anyone else have an opinions?