When you say "weasel out" you make it sound like the broker is trying to do something not allowed by contract law. This isn't proven.
The broker does not have to prove what the client was thinking: their position can only be based on what they can show the client did. This is the way good law works.
The client either. He traded on prices provided by the broker on the broker's platform and that's it. He made some profit and all of the profit he made is his. That's it. And that's the point.