Tell his ex-wife and ask if she is concerned about his intentions towards his daughters. This is about their safety, even more than about your nausea.
Rough one. I would guess it's just porn he watches and anything with his kids would not happen but since you don't know, agree telling his x wife what you found would be best to make sure they are safe and then you just keep moving and don't look back. This is not the guy for you.