I coach local high school teams and I also string for these kids along with many other local players of many levels. I generally prefer to avoid poly strings for anybody who doesn't really need its durability or performance, but I've found that the best first step for anybody who wants to try a poly is this sort of poly you're describing that includes a very light gauge of poly main paired with a softer cross string.
If this girl was using a full bed of 1.25mm RIP Control and the poly hybrid she tried left too many of her shots much too short in the court, it seems as though she might not really need any poly yet. Using a less resilient poly hybrid (less resilient that a bed of multifiber or syn. gut) might force her to over-swing too often to get her shots deeper in the other end of the court. That can spell trouble. I'd say first try to find acceptable control from an option among the softer strings before going into a poly.
A heavier gauge of RIP Control could offer her a little more control, but she might also find more control and consistency with her shots if she plays with a full bed of synthetic gut. Whenever I've played with RIP Control in the past, I've enjoyed its inherent softness, but that string also seemed to steadily degrade - soften - as I used it over a few sessions. Many multifibers seem to be prone to this progressive softening.
I'm a BIG fan of synthetic gut because it gives me moderate softness without degrading like many multis do for me. Better performance (for me), better service life, and significantly lower cost per set. I coach a girls' high school team and although several of the girls are very solid players in the age range of 14-18 years old, only a few of these girls use any sort of poly. Most of them use syn. gut. In fact, a couple of those girls using a poly don't even have strong enough technique to actually benefit from that string type... they just want that string because they're convinced that it's "better".
A few syn. guts are semi-stiff and several others are rather soft. My favorite syn. gut is from Kirschbaum, but I can only get it in 16 ga. While that's generally great for me, I also like to sometimes use a 17 ga. (1.25mm) option, at least in my own racquets. I keep some Babolat 17 ga. syn. gut on hand for that and I also use a lot of the hot pink 16 ga. Babolat syn. gut during the girls' season. Looks great, plays great, they love it.
Two syn. guts that are relatively stiff (too stiff for my taste) are Gosen OG Sheep Micro and Prince SG with Duraflex. While the Gosen is too clunky feeling for me in a full bed, I do often use it as a cross in many poly hybrids I install for the kids and other locals. It's durable and slick like a cross string in a hybrid ought to be to work well in that role.
