IMO, this rather sounds like e release problem, not a ball's. A stronger ball (with a lower RG) can help, but the weak hits might also be caused by low speed and revs, having the ball - any ball - roll out and hit like a wet donut. We have some female players in my club who are struggling with this, and I have hardly seen an improvement through ever more aggressive balls.
Without having seen her bowl, recommendations are hard to make, but the basic technique problem is something to consider. In that case, I'd rather seek some help form a coach, or ask from a seperate source beside the pro shop for help concerning her style.
Besides, many higher end ball under 14 lbs. weight are not avaliable at all or lack a true core - some use generic cores, sometimes you even just get a pan cake thing. Therefore, and if she feels comfortable with the weight, I'd stick with 14 lbs.
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DizzyFugu - Reporting from Germany
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