Sorry....The kid should have found out himself before he began cutting from the people whose lawn he is supposed to cut. He’s not a little kid chasing a ball. He’s destroying property.
I am totally serious. My brother and I always checked if we had a doubt.....that was in the 50’s.
What kind of lawn service - with other helpers/employees - is this kid running, where he doesn’t engage one or two of them to survey the proposed lot the lawn is on, and mark the corners, and string the boundaries? If he really is serious, run a chalk line.