Problem Downstairs Tenant Forced Upstairs Tenant to Move Out

Hello I have an upstairs tenant of 5 years and a downstairs tenant for 10 months. Due to tenant-tenant disputes, the upstairs tenant gave me notice that he is moving out at the end of May.

On the way out he told me that the reason he is leaving is because the downstairs tenant moved in a third person and is causing the utilities bill to increase too much for him (upstairs pay 60%, downstairs pay 40%). I looked at the utility bills and it's true, this winter was almost $500 vs $250 for the same period last year. Also his dog has been threatening him and his family such that they can't even use the common areas like the yard and laundry machine. And also other personal grievances between them that police were called.

So I dropped by the place a few days ago just to help clean up some stuff and make it easier for him to move, and when I got to the courtyard (common area) the downstairs dog rushed out and charged and barked at me. Luckily I had my phone ready and got a video of this, but obviously I couldn't enter until the downstairs tenant got the dog back inside.

What can I do in this situation? Thank you all

Edit. This is in BC Canada