Plant rose mallow (aka hardy hibiscus) or malva sylvestris instead, since they die down each year...no need to prune. I don't think ROS would be long term happy that way...though they are like weeds, and could probably be pruned to ground each year. Obviously, pruning will affect flowering (how early and how profusely)..given that, maybe she should plant dwarf boxwoods instead if she insists she needs 3-4 ft height.

You can keep pruning Rose of Sharon - some people have them as trimmed hedges in Brooklyn - but it's a fair amount of unceasing work, as they grow so much during the summer. Your friend may want to try the hardy hibiscus instead, as dbarron says, or another plant altogether, unless she doesn't mind the work.

Thanks, folks. Wanted to check so as to give her options.