Your plant is called a Fiddle Leaf Fig plant (Ficus Lyrata).
There are a few things can cause the brown spots on a fiddle leaf fig plant.
1. Overwatering is the most common cause of brown spots. If the roots stay wet and are not allowed to dry out a bit between waterings, brown spots develop (first on the older leaves). These leaves eventually drop off. The discoloration usually starts at the edges of the leaf. The cure for this problem is simple, cut back on your water. If your client is not sure how wet the soil is at the bottom of the pot, have her/him buy a water meter.
2. The brown spots may be the sign of a plant disease such as Leaf Spot disease caused by a bacteria that has infected the plant. These spots are usually lighter in color and generally appear on the newer leaves. The leaves may turn yellow eventually fall off. As with all fungal and bacterial diseases, better air circulation, well-drained soil, dry leaves, bright indirect light, and less water help control bacterial diseases on plants. Never mist a plant if leaf spot disease is suspected. You can use a commercial Fungicide
or the homemade remedy of putting a tablespoon or two of baking soda and a teaspoon or two of mineral oil in a spray bottle of water. Shake the solution well and then spray all areas of the plant that are infected. Keep infected plants away from your other houseplants.
3. Under watering: Pale brown, dry looking spots that start at the edge of the leaf and leaves that become soft and start to curl up are a sign of under watering. In cases of severe under watering, you may have to set the plant in a deep saucer of water for 15 minutes so it can absorb the water through the drip holes in the bottom of the pot.
4. If there is not enough light, older leaves may develop brown spots.
You can read all my care tips for fiddle leaf fig plant (ficus lyrata) in the Popular Houseplant section of the website.
These plants are considered poisonous and should be kept away from pets and children. The sap of a Fiddle Leaf Fig Plant is very irritating so use gloves when working with this plant. Read more about common houseplants that are poisonous in my book Don’t Feed Me To Your Cat! A Guide to Poisonous Houseplants.