My daughter has moved to a house where a very large Magnolia tree is obstructing light from bedroom window. Can it be pruned down by at least 4 feet without damaging it?
Quote (from the website of the Royal Horticultural Society):
"Heavy pruning of mature magnolias can induce watershoots (long, vertical, vigorous shoots) and dieback"
The sap rises in early spring with Magnolia and for this reason I would hold back on pruning til late summer.
To be honest it's not a tree that responds well to pruning, so it's best kept to a minimum but you can probably get away with losing 4ft off the height if done carefully.
Apart from that care should be taken to avoid any root disturbance.
I have an old one and I really wouldn't do any heavy pruning....A little each year is the best bet. However, if she doesn't mind taking a risk and wants the light back....then, I might be tempted