Do you get a lot of wind during the wintertime? I don't know if they'd make a good windbreak or not during their leafy season, but since they're deciduous there's going to be a chunk of the year where they don't do nearly as much for you.
My guess is that there are better choices for windbreaks (even when they do have leaves). I've never made a windbreak though so I don't have any great suggestions, but I do know that I've seen several other threads about making windbreaks around here recently--I think there was at least one question in one of these beginner forums, and another one or two in some of the regular forums--you might try doing a search and see if you can find those (if I remembered better where I'd seen them I'd post links for you, but since I'm not an expert on windbreaks I didn't pay too much attention to them)
They will break the wind some but not like trees would. You could plant the ROS then maybe a few feet away or in another location plant hedge trees (osage orange, hedgeapple). A little is better than nothing though so I would go for it.
I don't know how close to the coast you are--are hurricanes an issue? I lost my rose of sharon to Hurricane Rita. It was quite well established, but the prolonged winds simply laid it on its side. I tried resetting it upright, but it slowly began dying. By the next summer, it was gone. I don't think this shub would make a good windbreak.
Personally I would pick the best windbreak plants regardless of whether they have flowers or not, then plant flowering things in front of it. That way he'll have a good windbreak and something pretty to look at. I'm just afraid if he insists on flowering plants for the windbreak, he may end up with something that's less functional than it could be.
true windbreaks are a little tricky. Most are conifers that are at least three rows staggered deep and which allow some air to pass through otherwise if not done right it can actually increase the air pressure as they go around them. You're basically wanting to slow the air down, a buffer, so to speak.