Legal Question in Real Estate Law in California

They should be fully responsible, unless your line had already been leaking water so attracted the roots to it. Get the name of their home owners insurance company.

Unless there are other contributory factors, the neighbor should be responsible for all your costs, but if you can settle without suing for a figure that satisfies you, that's probably long-term preferable to taking your neighbor to court, even Small Claims ($7,500 limit).