I'd do it.
I know some people would think it too intrusive or accusational (is that a word?), but the results could be so serious that I would bring it up. "That's a terrific-looking poster. How did you get the rights to use Beatles lyrics? I've always heard they're really strict about giving permission."
She might shrug you off. She might say something about how friends should be supportive. She might get angry and say you're calling her a crook (not realizing that you're trying to keep her from being a crook). So be prepared; you'll need to choose in advance not to be hurt by such reactions.
But the idea is, without lecturing, to put the bug in her mind - to get her thinking. "Of all the nerve, for that woman to imply that I'm breaking the law!... What IS the law about that, anyhow?" Then it's up to her to take action.
It will have big repercussions for her business, but she won't have been the first person to assume (wrongly) that something so emphatically out in public is "public domain."