I took over care of a friend’s ficus last spring, it thrived during the spring and summer indoors. Over the winter it lost 80% of its leaves and is continuing to drop leaves. I tried adjusting watering, have adjusted its location so that it gets diffused light (east facing window) in the morning. Nothing seems to work. It has some new growth but overall seeing more decay than growth. The tree is about 5 ft tall, the pot is about a 13 inch diameter.
Leaf drop can be normal especially during the winter months but such a large loss of leaves is an indication of either some environmental factor or attack by a disease or a pest. Under watering and over watering can show the same symptoms especially when you have a plant in a large pot. A handy moisture meter probe that you push into the soil can indicate whether the soil is water logged or dry. You can purchase one online from a number of sources including many nurseries and hardware stores.
Another environmental issue is that the plant has become pot bound. Check to see if the soil is heavily compacted. Are roots showing on top or coming through the bottom. Try tipping the tree over and pulling it out of the pot to take a good look at the root mass and remaining soil. Removing the plant from the pot will also show you if it has become waterlogged or is too dry. Often compacted soil will prevent water from reaching the roots. The water simply flows down the inside of the pot and out the bottom while leaving the bulk of the soil dry. A Ficus benjamina should be repotted every 3 years. See the link below about repotting a potbound benjamina.
If the leaf drop is caused by pests, you will have to look carefully at the leaves and stems to see if you can find any sign of them. Here is a very good link that shows photos of various pests and disease damage and how to identify them.
Good luck with your plant.