My question involves real estate located in the State of: California

Husband and his ex divorced in 2003 - she was unfaithful to him. We married in late 2004.

She had agreed that in exchange for a lump sum of money she would execute a quit claim deed and remove herself from the title of our home. We refinanced the house after we married in 2004, gave her the lump sum and she executed the deed. I also executed one so that I was added to the title of the house, such that it was then just me and my husband on the deed.

I was shocked two years later in 2006 when we received a copy of a newly executed quit claim deed from his ex stating that she is back on the title and has a 50% ownership of our home and that me and my husband have 25% each.

How can she do this without us consulting us about it first?
Can anyone execute and file a quit claim deed - even if there is nothing to substantiate its truthfulness?
What can we do to get her off the deed - we already fulfilled our end of the agreement?

I'm worried that if anything happens to my husband I could lose our home to her. I really want nothing to do with her and this has been preying on my mind for some time. I have no idea where she is either...

Any suggestions?