Depressed or Distressed?

So my sister got this betta fish... It is a huge veil-tail male! He hasn't eaten in 3 weeks since she's got him! I don't know if he is sick or depressed. Even though he is not mine I feel so bad for him! For one, he is in a one gallon tank, no filter, no heater, and it is jammed with so much cheesey decorations he doesn't even have space to fan out his fins! Number two is, she does no water changes. At all! I end up doing the water changes occasionally behind her back! I tried to convince her to buy a heater and a small filter and to take out some decorations but she just ignores me! Anyways the fish is always in his "cave" and never comes out except for air. He looks at the food and ignores it. (I even tried brine shrimp which I feed my betta for treats!) My sister does not feed him unless I remind her so he is not full, just refusing to eat. He does not flare at his reflection or even my male next to him!