Certified GKH Gardening Expert
It likely has been overwatered. This will lead to what you see here. This bacterial blotch is quite aggressive, and will likely never go away completely.
You can control it, but eventually it will kill off the plant.
Control includes antifungal agents, and cutting off any HEAVILY affected growth. Cutting off too much at once will kill the plant, as well, so it is important to only cut it off once it is too unhealthy to photosynthesize.
This article will offer more information: https://www.gardeningknowhow.com/houseplants/hpgen/desperate-houseplants-dealing-with-common-houseplant-diseases.htm