what would have happened if millie hadnt truned montag in for having books?

If Millie hadn't turned Montag in, they might have gone on living in the way they'd already been living, or possibly Montag would have eventually moved on and joined Granger and the others regardless. There's also the chance, Montag would have been found out and eliminated without Mildred's help. This story could have gone in a variety of different directions.