Pruning Willow Tree Roots?

We have a neighbor next door that has planted 4 to 5 Globe Willow trees in her backyard which is not a large back yard, and also at least 3 more in her front yard, most of them on the east side which is next to our home. Several years ago, the city installed a very large underground sewer pipe which went between our side yards, under the street and between the yards across the street to their alley. She waters these trees constantly but since watering is allowed by our city ordinance, if you use a hand held hose, she is not out of compliance with the city. She uses one of the curly hoses that she curls around the trunk of the trees and leaves the water running even when she is gone from home. We have complained to the City on several occasions with no results.

They are unconcerned with the invasion of the roots into the sewer line. The constant watering has killed our grass and as a result we have had to replace the grass twice as well as having a French Drain installed. All at our expense. We absolutely do not know what to do or who to turn to. The man that installed the French drain tried to talk to her and she refused to discuss anything with him. Our chemical company that treats our lawn was here when the water was gushing out from under her fence and running down the street and flooding our yard. Between the constant flooding of our yard and the fear of the roots damaging our foundation, which we just spent $ 4000.00 to repair, we don't know who to turn to for help. Any suggestions?