As others have suggested, I'd start over. It doesn't look like they're are a ton of things to save and the tank is pretty small.
I wouldn't put too much blame on the house sitter necessarily. 6 months is a long time to put a tank in someone else's hands, particularly in they're not into the hobby. Even if they are, from my limited experience, different tanks may behave very differently and smaller tanks are tougher than bigger tanks. Now, why he didn't alert you things were going south, that's another question.
Good luck. Use it as an excuse to try something new/different. Then, you might actually enjoy the process.
Matt