It really comes down to your wife's bowling goals. If she just bowls to have fun and doesn't care about how she scores, just let her use whatever ball she wants. If it were me, I would probably have her stick with a plastic ball that she likes the looks of. The Nano would also work just fine for her in this scenario, but don't count on the ball providing any special benefits. If she throws hard and straight with no lift, the ball is going to go straight and hit pretty much like any other 12lb-ball.
Now, if she wants to take the game more seriously, then I would follow the advice of others. Invest in a lesson or two with a trusted bowler or pro shop professional. I would seriously advise you against trying to teach her yourself. For whatever reason, husbands and wives often struggle to get along when playing the roles of teacher-student. Let someone else teach her, get her switched from conventional to fingertip, and try to bump up the ball weight if at all possible to help with hitting power.
Again, decided based upon what she wants to do. If she just bowls for fun, any ball that fits will be sufficient. If she wants to improve her game, invest in some lessons, practice, and new equipment.
