U Trip Slow Pitch help

[Skahtboi]

From page 46 of the current USSSA Slo-Pitch Rule book:

1. When a batted ball, either Fair or Foul, is legally caught on the fly
while the Fielder’s feet are still within the established lines of the playing
area, the Batter is out even though the Fielder’s momentum may cause him
to fall over the fence, into a dugout; or patron areas, or cross a line, or
marking the out of play area, provided in the Umpire’s judgment the catch
was completed. The ball is declared dead and each and every Runner is
advanced one base after the catch. If ball is carried intentionally into a dead
ball area, two bases are awarded to each Base Runner.

From this citation it sounds as if your partner was correct.