The reader sees that there is a great deal of external conflict, especially between Lennie and various people such a Curley's wife, whom he ultimately kills accidentally. George, in his trying to take care of Lennie, often intervenes in the "man vs man" conflicts because Lennie's limited abilities causes him to miss the nuances of life among groups. George does have some internal conflict, since he has to decide whether or not to kill Lennie. He makes the decision to "put him out of his misery" which has not happened yet but would be "external" if he had to face the "mob" after Curley's wife's death.
There are multiple answers in mind but this one stick out the most inside of my head