Back at uni our landlord had to send out his maintenance team to unblock a drain in our back garden. Turned out the blockage was from two or three houses down and it was actually half a baguette, yes a baguette, blocking the drain :-/
try not to mention the tree roots thats probably the only straw he can clutch on
Its seems to me if the drain is damaged by your roots then its irrelevant what they have been putting down their drain.They are just going to use the damage as an excuse.
Where is the damage to the pipe?