Crab apple trees are fairly easy to maintain. Especially if they are well established as yours is. I would advise you not to listen to the advice you were given unless you have a specific issue with the growth of the tree. If the tree requires any pruning it should be done in early spring. Pruning now may result in less flowers and fruit later in the year and unless you have a specific problem or just want to thin the tree canopy you should probably just leave it alone. Here is a plant profile from the Royal Horticultural Society to get you started: http://apps.rhs.org.uk/plantselector/...