What George tells Slim about the incident in Weed is based largely on what Lennie told George while they were on the run from the lynch mob. George was not present when Lennie grabbed the girl's dress. Lennie regularly lies to George, and furthermore Lennie is mentally retarded and doesn't understand his own feelings, motives, or actions.
Steinbeck wrote his novella with the intention of turning it into a stage play almost immediately. According to the Introduction in the eNotes Study Guide, the play came out in New York the same year the book was published. The book is a bit unusual in that most of the exposition is conveyed through dialogue rather than narrative prose. This was obviously intended to make the adaptation to a script form for the stage play quick and easy.
The incident in Weed is very important. Steinbeck has George rehash it with Lennie in the opening chapter, and then he has George repeat it with embellishments to Slim. But the fact that George confides in Slim is not especially significant. Slim is just the most intelligent and the most discreet listener available. George trusts him. Slim may read more into what George tells him than George himself understands.
What really happened in Weed? Lennie went up to a strange girl on the street and grabbed her dress. He told George he just wanted to feel the soft, smooth material. This was probably a lie. Lennie is developing a rather delayed interest in sex which he doesn't understand and can't control. He was probably more attracted to the girl than the dress--but he doesn't tell George that. And George doesn't realize the truth until he sees Curley's wife lying dead in the barn. Then he says to himself:
"I should have knew," George said hopelessly. "I guess maybe way back in my head I did."
George wasn't present when Lennie killed Curley's wife, either. It looks like an accidental homicide in connection with an attempted rape. And that is not very far from the truth! Lennie might have raped Curley's wife if she hadn't started screaming and struggling.
The Weed incident plus the killing of Curley's wife show a definite tendency. Both are related to Lennie's unusual interest in soft little animals, which he always ends up "accidentally" killing. Lennie is becoming a menace to society. George can no longer protect him. He helped Lennie escape from the lynch mob in Weed. He could help him escape from the lynch mob led by Curley. It would be easy, because he is the only one who knows where Lennie is hiding. They could wade across the shallow river and lose themselves up in the mountains on the other side. But George has no intention of helping Lennie this time.
*All quotes are taken from the Penguin version of "Of Mice and Men" (1993).
George tells Slim that Lennie got in trouble in Weed because a woman accused him of rape:
"Well, he seen this girl in a red dress. Dumb bastard like he is, he wants to touch ever'thing he likes. Juse wants to feel it. So he reaches outto 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, that girl rabbits in an' tells the law she been raped. The guys in Weed start a party out to lynch Lennie. ...But he never hurt her. He jus' wanted to touch that red dress..." (Pgs. 41-42)
George tells Slim the truth about what happened in Weed; Lennie touched a girl's red dress and she falsely accused him of rape. In turn, they were chased out of town.