Well, assuming you know how to hook the ball properly, I'd say stick with one ball until the 160 average range, I know that seems really low, but I think a bowler should have two balls, maybe a third if they are going to use plastic for spares(chances are though their spare game will be weak). I think have the two balls will help a new bowler see the difference in ball reactions, and see lane transitions and such. It just needs to be made sure that having two balls, that adjusting becomes just switching between the two, got to move your feet/target.
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-If you're not willing to change, you're not willing to improve.
-Life is like a coin, you can spend it any way you want, but you can only spend it once.
-90 percent of the game is half mental
-Yogi Berra