Regardless, the time to do something about it is now, and it starts by being informed. Getting informed doesn't mean being told it's good, it's bad, it's advanced for that amount of time, it's hopeless, it's realistic, it's impossible. You start with some concrete things so that you know what is up and what is down. When that happens people also reform their goals. Just as important, they reform their approaches. When that reformed approach happens, maybe in the long run they won't plan to become a professional pianist along any venu. But they might end up being a rather decent player.
A stiff arm or a fluent playing style don't just happen. These are things that are taught and watched for. OR a teacher may decide to give increasingly difficult pieces to give a feeling of advancing, and then stiffness and all kinds of things creep in. The very first thing is to define your goals and ask how to get there, of the teacher. Many adults just sort of "fall into" lessons and all kinds of things can happen. The very first thing is to get informed, find out. From being informed, the goals can change too. Saying something is impossible will not make that happen.
I'm afraid I don't have time to organize my thoughts more. This will have to do.