Unfortunately, dogwoods in general are pretty susceptible to leaf spot. But as you know, leaf spots are on the plant, not in the plant. So this fall, when the foliage drops, they should clean it all up and discard it, which will reduce the potential for reinfection next spring when the foliage emerges and has not yet developed its protective cuticle. Also, if it is too closely spaced to its neighbors or nearby buildings, they should consider moving it – good air circulation goes a long way toward mitigating leaf spot. Fall or late winter/early spring would be a great time to do that. Finally, if they have a sprinkler system, I would suggest they adjust the heads to avoid wetting the foliage when it waters.