My daughter teaches piano. Parents flake on her all the time and sometimes fail to pay. My response to her is simple. It's her fault for not having a contract and for not charging up front. I tell her she needs to change the way she does business. Same thing with your boys. This is between them and the neighbor. First off, they need to stop mowing the lawn. Second, they need to be paid in advance going forward. They best thing you can do however, is to let them handle it... even if it means they lose out on some money.