I've been there and it is convenient and the closest option since the Collin County and Backwoods ranges closed down. The times I have been there they have been well supervised- no complaints.
Behind the berm are a lot of trees. It would be possible, however, to launch a round at a pretty high angle.
But how could they ever prove it came from there? Without removing the bullet from his back, there can be no ballistics match. The range would have records of who shot there that day (June 12), but being a Saturday I'll bet the range was full. So then how could they possibly find out what every person present was shooting- down to the ammo?
The suit is more about coercing a settlement than anything else.

I know FMJ is prohibited, but my question is why is it prohibited. It could be banned for no good reason.

In a civil suit, they don't have to prove beyond any reasonable doubt, they only have to convince a jury that it's more likely than not that the bullet came from the range. That's not a very high hurdle unless most of the folks in the jury box wear cowboy hats.