Claiming liabilites
Can someone wanting to file for bankruptcy claim a co-signed debt as his OWN liability?
It becomes the co-signors debt anyway, so wouldn’t the co-signor have the right to claim it as HIS own liability since HE is the one guaranteeing it in the first place?

While I am not clear on what is meant by a “claim a co-signed debt as his OWN liability,†I think what you are getting at is whether or not this should be listed on bankruptcy documents, and the answer is yes. You are responsible to list any debt you are responsible for, regardless of whether or not someone else has guaranteed or cosigned on that debt.