No. It is not still married and according to widowed grandparents, there is nothing formal to dissolve a marriage.
I am not a tax or Canada Revenue expert, but I had similar issues with a Divorce. I was considered married when I wasn't and had written divorced for two years and needed to send them a copy of my Certificate of Divorce so show I was no longer married to stop their requests. My response to this request would be to mail them a copy of the Certificate of Death. Obviously someone wasn't reading when they came across this request and put it into the system as Married when its not or your friend got one of their "random" requests for additional information to ensure that people aren't lying to them. When they see the Death Certificate dated from the 80's, that should answer their questions.
There is a form,
http://www.cra-arc.gc.ca/E/pbg/tf/rc65/rc65-11e.pdf That you are supposed to fill out and send in upon change of marital status. Maybe include this as well so Revenue Canada has their paperwork filled out with the information they need.