You might want to pick up one of these or these for your mom. It might take some tweaking to get it set up right so that it doesn't soak innocent passers-by, but they're adjustable and you should be able to get them to only detect and spray things on the lawn.
Yes, your mom is guilty of a minor infraction, as is her neighbor. I would be more concerned about retaliation than about the police doing anything about it.