Does my parent’s new partner have any rights to their assets if they pass?

This question has many variables that may affect the legal outcome. However, to answer the question here we will assume that: 1) your 83-year-old father is formally divorced from your mother; 2) there is external support for the conclusion that he and the 74-year-old woman are not relationship partners, i.e. they do not hold themselves out as a couple, they do not eat all or most meals together, they do not constantly socialize together, etc.; 3) your father has not changed his Will to include this female roommate; and 4) this scenario takes place in Ontario.

We will also assume that the verbal agreement to “try things out for a year” relates merely to the financial aspects of the living situation, meaning it is an attempt to formalize their respective obligations around shared expenses and paying bills relating to your father’s home.

With these many parameters in mind, the short answer is this: Assuming that this is a strictly platonic “roommate” situation, with evidence to verify that conclusion, it is unlikely that a court will find that the roommate has any entitlement to your father’s home, assets and pension after his death.

However, the answer may change entirely with even slightly different facts. It is important to get the tailored advice of a family lawyer in your situation.