FWIW, there's some evidence that shorter radius skis may create knee unfriendly conditions when they're backweighted. Cannot speak to boots except to say that using an overstiff boot may be just as bad as an understiff. The flex needs to be married to the forces she's generating, which means her weight, her typical speeds, and her edge angles. Also assume she's doing PT, make sure she builds up her hamstrings. They stabilize the knee, especially important to the ACL, and they tend to be less developed in women relative to their quads; this imbalance has been cited as a risk factor in ACL.
As much as I'd like to support Finn here, I'll have to add that everyone who suffers a significant knee injury develops degenerative OA. Good sized lit on this, has to do with molecular level, not whether your surgeon did a bang up job. Very small scale anatomical remodeling begins pretty quickly, although obvious functional changes may take a decade or more. If you're middle aged, everything takes longer to heal and is less likely to heal perfectly. That kind of stuff, not the ACl being "weaker," is going to be the organic issue. So I'd suggest a ski that can absorb some jolts for her would be a good thing both for her knee and her mind. Head, Kastle, Stockli, Rossignol come to mind as brands that one way or the other manage to build smoother skis. Something along the lines of a Temptation 82, Kastle LX82, or Head REV 80 might work well. Trekchick may be able to give you some solid advice on additional women's models that are appropriate for an intermediate.
Finally, would underscore the need for lessons. Right away. Will help her rebuild confidence, and reshape her technique to reduce the chances of doing it again.