Taxes in Canada, unlike the US and some other jurisdictions are always filed separately. There is no joint filing; however you are expected to declare your spouse on your tax forms.
The primary reason to declare a spouse is to claim a spousal amount if that person has no (or very limited) income.
Some help about what a spouse is from the Canada Revenue Agency can basically be summarized as living together for a year, or that you have a child (natural or adoption) together.
If you had been living together for some time, then one passed away, for that given tax year, I expect you could claim some spousal amount. But if there are deeper issues you need to clarify, then a visit to a tax lawyer would probably be worth the cost.