Hi Mago:
I think the only thing your wife has to do to "keep her curves" is not let the body fat get too low (and for most people, inadvertently getting body fat too low is not very likely). If your wife has been heavy most of her life, then inadvertently getting too lean is just not very likely to happen, and basically neither of you should waste time worrying about it.
I think your wife should be able to do just about anything weight-training wise that she wants to in terms of weight, reps, sets, etc without worrying about getting ripped.
In my 30's I weight trained very seriously, my goals were increasing limit strength so I often used heavy weights/low reps. I never got ripped because 1) my body fat was never below 20% or so and 2) like most women, my muscles don't bulk up regardless of my training regime. Also, the size and shape of one's muscles is mostly genetically determined. You can't really control it very much.
As someone else mentioned, however, loss of flexibility is a real problem with weight training, unless you step up the flexibility training as well.
As a dancer and a weight trainer, I think that the most useful weight training exercises are compound movements that build coordination, flexibility, and core stability, using free weights. For example, free squats, emphasis on perfect technique. Doing isolated exercises on nautilis equipment isn't very useful for me.
You might find this website helpful. It is the best on the internet for women's weight training:
Women's Weight Training