Let's ignore the house edge for the sake of simplicity. If the player’s two stopping markers are a win of $5,000 or a loss of $35,000, then the probability of reaching the winning marker is 7/8. The probability of doing that during 30 consecutive trips is (7/8)30 = 1.82%. So, this could easily be just good luck. I’d let him play until you determine why and how he is beating you.
This brings to mind a very good book I just read, Casino-ology, by Bill Zender. The main thrust of the book is that casino management is much too paranoid about advantage play. The overreaction to it is slowing down play and annoying legitimate customers, the cost of which is much more than what is saved by catching a few extra advantage players.