I had the same questions you have.
The grandfather will not be making the payments, and will not be on the title to the home. He just offered to sign, and the lender thought as an abundance of caution, strengthen the loan, etc., it wouldn't be a bad idea.
The DTI is a little high, but it is not so much the loan would be denied. It will have to go through ATR, and the co-borrower/director has a plethora of assets. The lender was trying to avoid collecting and verifying all that, for a $40,000 home loan.
To answer the last question, the lender knows if the borrower does not pay, the co-borrower/director will.
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Just when I think I understand....I regain consciousness!!!