What about Garlic Barrier? We’ve used it to repel deer and bugs in our garden and yard. It lists flies as one of the things it repels. You could email from a site that sells it and ask specifically: http://www.biconet.com/flyers/gb.html
I swear by garlic barrier, but another thing that would help would be to take a bucket of water and “wash” off the area where the dog deficated or urinated to help dilute it. Your friends could also make a tea tree spritz to put on themselves when they go outside.