Why do you think George told Slim about what happened in Weed?

George told Slim the truth because he trusted him.

"Well, he seen this girl in a red dress. Dumb bastard like he is, he wants to touch ever'thing he likes. Just wants to feel it. So he reaches out to feel this red dress an' the girl lets out a squawk, and that gets Lennie all mixed up, and he holds on 'cause that's the only thing he can think to do. Well, this girl squawks and squawks.
I was jus' a little bit off, and I heard all the yellin', so I comes running, an' by that time Lennie's so scared all he can think to do is jus' hold on. I socked him over the head with a fence picket to make him let go. He was so scairt he couldn't let go of that dress. And he's so God damn strong, you know."

Slim's eyes were level and unwinking. He nodded very slowly. "So what happens?"

George carefully built his line of solitaire cards. "Well, that girl rabbits in an' tells the law she been raped. The guys in Weed start a party out to lynch Lennie. So we sit in a irrigation ditch under water all the rest of that day.