If your mother purchased said property as "primary residence" , she signed 2 documents - promisory note and mortgage. Since she was married to your dad at the time, most likely he was required to sign the mortgage too, not the "note". Your father is responsible for the terms of the mortgage.
The best way to find out is if your dad requests a copy of his credit report . If the loan in default was reported on his name too it will show there. If it doesn't show - he would not have any problems to obtain a mortgage providing that he meet all other qualification criteria.
As a consumer he can get a free credit report from all 3 agencies ( and it will not show as an inquary) by visiting www.annualcreditreport.com
Please vote if my answer was helpful.
Iliana Ivanova P.A.
Real Estate and Mortgage Broker