Neighbors Barking Dogs, Need Help

I have a neighbor, who is also a close friend, who has two (loud) barking dogs. They bark at everything and anything and sometimes will sit and bark for hours at a time. Their invisible fence allows the dogs to roam and sometimes come close to my property and far from their home, but no matter where they are we still can hear them. They have been using bark collars with limited success and are trying to solve the problem but there seem to be very few workable solutions. They are a busy working family and are sometimes away from their home for many hours and sometimes days. What are their options? Are there any better types of bark collars or something else that would help?