You can try, but short of him telling her to buzz off and leave her alone, it’s probably not going to have the desired effect. She’s just not going to understand what she’s doing that’s wrong.
Really though we all have to deal with unwanted attention from people that we wouldn’t ordinarily interact with in public places. I’m a regular at a bar, and whenever I get into a deep conversation with a pretty girl, invariably there’s some jackass that wants to interrupt us because he wants attention from the pretty lady. You just learn to suffer the fool because it makes you look bad to aggressively assert yourself. And he’d have a point. It’s a public place. If you don’t want to be interrupted, go somewhere more private. Boom, problem solved.