Is it really dry where you are? Tree roots such as willows can run for '20 miles' to find water. Trees do not always depend on their main roots for water, they have 'quick roots' that the tree can grow to get ground water when it is scarce. Her garden and lawn are getting water, I assume, and trees can 'smell' it in the ground, chuckl, so those roots are spread out just under the surface of the ground to get that water. It shouldn't kill the lawn, but can be a problem in gardens.
