Love that you have one of your Mom's favorite flowers. Summer and Fall is not the time to prune Abutilon or Flowering Maple. Just let it grow and bloom now making sure it gets plenty of sunshine, water and fertilizer. If you see buds it should be covered with flowers soon. In the spring once the weather warms and the plant can stay outdoors that is when you can cut back that one stem if you want to encourage more branching and a more compact, bushy plant. In warmer, frost free parts of the country the Flowering Maple may survive outdoors all winter.  If brought indoors to protect it from freezing you see the leaves turn yellow and fall off don't panic. It is just going dormant during the winter months. As the days grow longer in the spring it will resprout. This is when it is safe to prune back by one half to one third to encourage branching. Keep growing, Marianne Binetti