I think it all depends on why he purchased the locomotive.
Did he purchase it to run the locomotive?
Did he purchase it to take it out of the box and display it?
Did he purchase it to have something rare and difficult to find?
He may wish to just leave it in the sealed box and have the pleasure of knowing that he has such an unusual item.
If he decides to open the box, and just display it, I think I would leave the screws alone. Any damage is already done.
If he decides to run it, the shell is going to need to be removed to lubricate the engine.
When replacing the screws, I would be careful not to over tighten them. I would think about making rectangular shims to go between the body and frame to take up the gap.
Honestly, if the engine were mine, I am not certain what I would do with it. Over the years I have purchased some Postwar Lionel sealed box items. So far, I think I have opened all of them.
I expect this thread and my comments will draw the usual responses about crazy collectors and people putting bricks in boxes.