if silver raced Du and he won, (by cheating) and Du knew he was cheating, what would Du do.

I'd most likely pretend to not notice and go up to to "congratulate" him and instead of shaking his hand I'd schlagen, punsch him in the gut then slap him saying "take that cheater!"

do nothing and congratulate him...

Silver wouldn't cheat in a race, I'm sure about that.

I would rematch him in a different way to race. Then challenge him to do what he did to cheat without it.