My girlfriend and I have been in a couples league for about 4 months and she has been using a house ball.  I wanted to make sure she wouldn't lose interest before I went out and bought her a ball so it wouldn't just sit in my locker (and to make sure she was still my girlfriend, haha).  I am now convinced she will use the ball because she enjoys bowling.  She gets frustrated because it is hard to be better than a spare bowler with a house ball.  I just recently purchased a more reactive ball and have enjoyed the small spike in my average but I bought the ball that the owner of the lanes recommended for me.  I have no reason to doubt his judgement but would like some outside opinion on what type of ball to get.  She feels most comfortable using a twelve pound ball. Any advice would be appreciated, sorry such a long message. Thanks!

Think Cheap.  She can't go wrong with a Power Groove or Tropical Storm.  If you want real cheap, get her a Urethane Scout from Columbia.  You can find them on eBay for about $30 shipped.  Remember, you're most likely (and I say most likely because nothing ALWAYS applies) going to see her bowl better with a ball that is actually fitted for her hand.
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Formerly SGC300

The ebonite Tornado is a good bowling ball as well, for a beginner and stronger than you would think. Just another to consider as well as the Storm Tropical. If you are bowling on heavier oil than an Ice from Ebonite would be good, or a Heat Blast from Track. Roto Grip also makes wonderful stuff at the lower end of their line. A Mercury, or the new Neptune would be a good ball for the lighter oil possibilities. For medium oil conditions, get a Horizon.

Chuck