Couple wrecks trailer 20 minutes after buying it

1) The weight due to 2x4 floor is likely irrelevant. If it was a newly purchased unit, it was most likely empty and a few boards are unlikely to put it overweight. If anything, that's a lot of weight down low which would help with stability.
2) Unless they have clear evidence that the dealer was responsible for setting up the tow combination, dealers generally don't take responsibility for that.

This sounds like a typical newbie mistake. It started swaying and she likely reacted poorly making it worse.