As you might imagine, a situation such as yours is complex and difficult to separate into its component parts. There are no legislative guidelines to help determine the rights of what can loosely be called “landlord-landlord disputes.” These matters tend to be determined in the courts, and presumably the family court involvement may end up determining what rights each of you have in the context of property ownership, and in the landlord and tenant relationship. We only give advice to the small-scale landlord community. However, my own assumption in this matter is that if the tenant was to get legal advice on the issue, he would be told to deal with your ex-spouse exclusively, and to consider you to be a neighbouring tenant, rather than a landlord. This stems from the fact that your ex-spouse is the sole person who actually showed and rented the unit to the downstairs occupant, (presumably) charged him, and continues to charge him rent.

Section 2(1) of the RTA defines landlord as the following:
“landlord” includes,
(a) the owner of a rental unit or any other person who permits occupancy of a rental unit, other than a tenant who occupies a rental unit in a residential complex and who permits another person to also occupy the unit or any part of the unit,
(b) the heirs, assigns, personal representatives and successors in title of a person referred to in clause (a), and
(c) a person, other than a tenant occupying a rental unit in a residential complex, who is entitled to possession of the residential complex and who attempts to enforce any of the rights of a landlord under a tenancy agreement or this Act, including the right to collect rent; (“locateur”)