It can be tricky as you note. The tenant has terminated the tenancy so in essence the sub-tenant has no rights of occupancy. They are now, in effect, an unauthorized occupant. An A2 Reason 2 is the defacto path.

These situations are fraught with land mines such as delaying too long before filing, accepting payment, etc. Not to mention the excessive delays at the LTB and requirement for procedural strictness within the forum. I highly recommend using a paralegal in a situation like this to ensure everything is done properly and efficiently minimizing any loss you may have. They may also be able to facilitate the sub-tenant's departure.

Feel free to reach out to me directly and I can put you in touch with a solid paralegal.