It's one year for Social Security spousal survivor benefits, see
https://www.ssa.gov/OP_Home/cfr20/404/404-0330.htm (10 years is for divorced spouses)
There are certain situations where it can be even less than a year, see
https://www.ssa.gov/OP_Home/cfr20/404/404-0335.htm and
https://secure.ssa.gov/poms.nsf/lnx/0200305100 but they are not likely to apply to OP's dad's situation.
Other factors to consider for Social Security:
* is there common law marriage where they live? if so, do they have one?
* is half of his benefit bigger than hers?
* how old are they? when did they claim retirement benefits/
* was she married before? how long did it last? is she eligible for benefits on her ex's record and is that more or less than she'd get from this man?
* are there any minor children or adult disabled children?
Inheritance is a separate issue and state laws vary. If there's enough money to care about, then there's enough money to pay a lawyer for good advice. Other things to talk with a lawyer about could be medical and financial powers of attorney (who makes the decisions if FIL is incapacitated?), living will (what does FIL want if terminally/critically ill?), and Medicaid eligibility.