I don’t see the problem. It’s not like she left with him. And if they did go to lunch (at the proper time), so what? It is none of your business unless you specifically have a policy against employees dating clients.
It’s a really bad idea for anybody at a law firm to date a client, though that didn’t even cross my mind. I guess I’m really in the minority in my position - oh, well, it’s a FRee country.