Vodka is perfect! Some use a sanitizer mix, and some use plain water. Those are all fine, as long as you remember that sometimes the liquid in the fermenter can be sucked back (via vacuum pressure, like in weather changes) into the fermenter. So, anything that would be relatively benign to your beer would be a good choice. Don't use bleach, for example.
He has a fruit fly problem and they can get in the lock through the little holes and lay eggs. Then the next generation gets in your fermentor while they are maggots. I have seen this.