Why does George Mcfly employ Biff to wax his car after he had previously tried to rape his wife?

In the book version, it makes it clear that Biff was, indeed, trying to rape Lorraine. When he sees her in the car after threatening to take the damage to his own car out of Marty's ass, he says (in the book) "Maybe I'll take part of it outta your ass."

The book then goes on to say, after Marty is locked in the Starlighters' car trunk, that Marty was kicking himself, figuratively speaking, for the fact that "if it hadn't been for him, Lorraine would be inside enjoying the dance instead of having to fight to avoid being raped".