Okay, so here's the problem my fiancé is having:

He and his then girlfriend were going to move into an apartment together, for 12 months. He signed the lease, but she did not; she signed a copy of the lease, and faxed it over. The landlord said she would have to come down in person and sign it, but she never did because she never moved in. Two people had to sign the lease, because he could not afford the place on his own. He stayed for 3 months, but could not afford the rent, so he was kicked out. Now, he received a letter from the landlord, saying he is going to need to pay the rest of the rent for those nine months (almost $10,000!). I have a few questions:
Is she at all liable for the amount owed, since she did at least sign a copy of the lease, showing her intent?
Is the lease even legal if the second required person did not sign the original?
Is there anything my fiancé can do to get out of this, or at least not get totally screwed by some girl that left him?

By the way, this was in California. Any help would be greatly appreciated.

The second person is not required to make the lease legal, once it was signed and accepted it was legal.

The landlord merely had a rule that the renters had to meet certain guidelines, they are free to allow exceptions. Since they allowed them to move in, it became a legal lease once signed

You did not say where this is, but a faxed or emailed copy that was signed is very legal, they did not have to sign the same lease agreement, just identical ones and together the two copies are the lease, obligating each to payment.

The landlord can come after both people, or just one, if one has more money.

As stated above, the landlord can go after either party for the whole amount. Your fiancé would have to sue her if he is held liable for it all.

Most leases have a clause that makes either or both tenants individually responsible for the lease. So, yes, he could be held responsible for the whole thing. As noted, as long as the landlord has something signed then the lease is valid.

If he is held responsible then he can turn around and sue the former girlfriend for her share.

But this is a big If. You say he was "kicked out". If he received a written notice to vacate or a court issued an eviction order, then his obligation should have ended when the landlord took possession. So he would only owe until that point. Also the landlord was required to make a good faith effort to find a new tenant. If he did not the courts will not award him rent while the unit was vacant. So, if this comes to court he should subpoena the landlords records on what was done to find a new tenant. Also check vacancy rates in the area. And, of course, check if the unit was rented to someone else.