If the wife wants to friend the dead husband, I don't think it's so important "what he would have wanted."
They should prioritize the wishes of the next of kin, or executrix of his will, or whomever our traditions and legal system has figured out can make this decision.
We as a society came to these practices over many generations. They exist for a reason, and the way this is phrased, it seems Facebook is putting themselves in the position of reinventing the wheel for whatever reasons they have. Which may or may not be in the interests of the survivors of the deceased.