Good luck with claiming on your window cleaning liability insurance if their roof starts leaking and they come after you.
Vin
He said in his first post that the customer asked him to do it, Vin. If this is exactly what happened, then surely the customer takes responsibility. I'm not sure that a customer would know his tiles need trimming back and was waiting for someone to come along and do it.
However, if Scottish advised them to cut the tiles back, and they then confirmed that he should do that, then he has to take responsibility for the advice and subsequent damage.
I can imagine a war of blame pointing in the future when any subsequent water damage occurs.