My mother received a 1099-C for the tax year 2008, on a cancellation of debt referencing my father's SS #. He died Sept 2005. The amount is > $20K. There was no estate to settle.

Is my mother required to file this under her SS# for income?

It depends on what the debt was for and if your mother was a co-signer in the debt.

See: Tax Topics - Topic 431 Canceled Debt – Is it Taxable or Not? (http://www.irs.gov/taxtopics/tc431.html)

It was a credit card for charges unknown as of now. This was his credit card and she was not on the account.

She has no obligation to report that.

She is now single and it has nothing to do with Social Security.