Yeah, it's hard to know what to do. I am not sure our local rules allow spectators to assist or observe. If both parties agreed, fine. If not? Don't know.
When I play people who cheat like that, I do not change anything about how I am playing. I don't aim away from the lines or hit down the middle. Then I have changed my game, which will work to their benefit.
If they want to cheat in a league match that badly, then I'd rather they cheat me rather than my losing fair and square because I was trying to change my game.
-- Random Error Generator, Version 4.0
-- Master Moonballer