Q.Why would a crape myrtle drop all its leaves overnight?
My mother’s crape myrtle mysteriously dropped all of its leaves overnight. Have you ever heard of this? If so, what might be the problem?

Crepe Myrtle is a deciduous tree and early Autumn weather will trigger leaf drop.
If the tree drops it's leaves outside of the Autumn time frame, it could be a fungus issue.
If you suspect fungus and not due to Autumn weather, rake up all leaves and dispose.
Spray the tree with Neem Oil. Use a 70 % solution and add 2 Tablespoons to a gallon of water. Spray the tree throughly.
Air circulation is key to good tree health.
Here are some links for you with more information.
https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-care.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm