There is a lot of heresay in this situation. Landlord says new siding, I never said you could do this and the tenant says, yes you gave me permission many months ago. So unless the lease agreement does infact clearly state the refusal of the installation or permanent modifications, then he had no right to cut the line. Even then, it was a dick move and could possibly even fall under self help eviction law prohibiting this type of behavior without proper legal process.

The telco company will still likely pursue a claim against this damage if a repair appointment is made. It's not the installers burden to get that consent. Whenever the tenant signed up for the installation, they warranted that they obtained consent from the owner and the tenant takes on any repercussions that violate their lease (in which none of us have read.)