Legal Question in Real Estate Law in California

No.

NO. The mortgage is different from having an ownership interest.

No. Ownership of property and liability on a loan secured by the property are unrelated concepts. Only owners can sell, and only what they own. "He who sells what isn't his'n, must buy it back or go to prison."

Well, actually it depends. If the person has an ownership interest in the house, then they can sell their interest. Your post is not clear as to whether the person co-signing has an interest.

There is another set of rules that may or may not apply, depending on whether the person co-signing is actually a guarantor. In that situation, a guarantor may pay a defaulted loan, and may have the right to foreclose against the owner of the property, when seeking indemnity.

Not unless he is on title as an owner.