First, what exactly is your problem? If it is her fault, he is not complaining. If it is his fault, she doesn't seem to be complaining either, and she could.
So lets explore some what ifs. If it were two people in the office, equal level, crawling all over each other, pawing each other, etc, would anyone care except for the occasional prude who would be upset it they were outside on a park bench.
Do you care about it because it is a public display of attention, or because it is at work, or because she works for him.
Are you afraid she is screwing her way to the top or at least a good review?
Unprofessional is the companies problem. Uncomfortable is your problem.
I had a boss once that was screwing his secretary. He was caught doing her on the top of his desk. The secretary was fired. The lesson is that you might get your coworker fired, and piss off your boss but I don't think you will change much.