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.