No matter the situation, one can always walk away, but there will always be consequences as well. This question is probably best suited for an attorney. Since most people who respond to these questions are not qualified to render legal advice (myself included), I strongly recommend that you discuss this matter with a lawyer.
I am not sure why anyone would take over a mortgage without their name being on the deed. With that being said, your best bet would be to contact an atttorney for legal advice.There are always ramifications when you sign for a loan and then you don't pay..