What evidence is there that Slim understands what had really happened on the riverbank ?

From the text:

Slim came directly to George and sat down beside him, sat very close to him. "Never you mind," said Slim. "A guy got to sometimes."

Slim also quickly removed George from any further questioning by the other men.

Slim twitched George’s elbow. "Come on, George. Me an’ you’ll go in an’ get a drink."