As far as I know, an ex-wife can't get any assistance from the VA. I would call the VA directly or look at your dad's will to see if he provided for her.
Being married at time of service does not count. If ur divorced you cannot apply for A&A. When she filed for SS did she tell them she had been married to him for 20 yrs. It would effect how much SS she received.
https://www.agingcare.com/questions/ex-wife-of-a-now-deceased-veteran-what-benefits-am-i-entitled-to-such-as-home-care-432566.htm