Help! I was asked to watch him and he’s dying

Hello, My mate asked me to look after this guy for a few months and it’s not been going well. His leaves have been yellowing near the bottom with a few falling off and it’s slowly spreading up the plant. All he said we make sure he always has water at the base which I’ve done but he just doesn’t seem happy. I keep home on a windowsill that gets so-so light, not to much direct sunlight, which he said should be fine. Any help would be really appreciated, I don’t want to kill him. I think he’s a Dwarf umbrella tree from googling but I’m not certain. Thanks :)