I don't know what to tell my gf about her boss.

You might think I'm joking, but at the same time, I hope she realizes that a lot of people do look at pornography. It's almost human-nature to do so, and some that will not tolerate it would be people that are of a religious background. Whatever weird sex videos or fetishes the guy might have, as long as he's a good employer and is a good guy 'all-around', I guess that's what matters, right?

Try telling her that ^, or find a better way of doing so. Because really, I can't see what you would be able to do at this point. You can either report the guy (which is sorta mean), but does he really deserve to be told on? You guys would have to choose.