If the situation is as disgustng as you say in that house, it would be a clever and responsible idea to call the authorities.
People that treat animals with neglect should not be allowed to have them. Period.
The fact that you know and do not do something about it makes you as responsible as them, you should think about the animal's interests before your frienship.
And honestly, I would be ashamed to have friends like that.
No offense and I do not want to flame or start a discussion, but honestly, someone who keeps a snake in a tub of mold and sh*t is a monster.