Maybe you have bought all the gear by now, but just in case--
Like some of the other folks here have said, women often sleep colder than men, and also everyone is different. For me, the most important gear choices are about sleeping comfort, and for me, that is all about staying warm and being able to sleep in the same sleeping positions as I do regularly at home.
For both of these things, I LOVE my Montbell UL Super Spiral Down Hugger sleeping bag. It expands to whatever position I am in, which in addition to letting me sleep in a normal position, also means that my knees etc are not pushing the bag to its max width thus making a cold spot where they are touching the bag. Then, the bag sort of contracts back again to snug around me, thus reducing cold air pockets. I use the #3, PLUS puffy insulated jacket and pants and footies, for the lower end of the temperatures you describe. Unless your wife is a warm sleeper, perhaps she would like to start with version one step warmer,the #1. I sometimes use it unzipped down to about knee or calf, to resemble a quilt.
I also have a Western Mountaineering Sycamore and love that too; the semi-rectangular shape allows for tossing and turning, it is a little warmer than my Montbell #3, and can be unzipped all the way to be a square blanket. Not everyone needs to toss and turn, though.
As for size, it helps to look at girth dimensions as well as length. Personally I find that the "hip" girth is not nearly as important as the knee girth (which is usually not listed) but that is just because I stick my knees out.
Another option is to get bags that you can zip together to share body heat, if you will not keep each other awake by tossing and turning.
Finally, for me, staying warm at night is in large part about getting warm BEFORE getting into the sleeping bag. I put on my sleeping clothes as soon as I stop sweating after arriving to camp. Puffy down or synthetic garments are most effective, for me. If I am wearing non-puffy sleeping bottoms, it can help to have an additional layer of something over the seat area-- and puffy socks are also fantastic! (Can be made by cutting sleeves off an old or cheap secondhand light puffy jacket and closing off one end with a rubber band.) But if your wife has something in the temperature range of the #1 montbell, she may not need all of that.
As for sleeping pad, if the thermarest is not comfortable, maybe the small size of one of the Exped pads would be good. I tried a small Exped Downmat UL7 and a regular size thermarest Xtherm (same form as the Xlite) at the store, and the Exped felt nice and wide comparatively, even at the places where their widths appeared to be the same according to my measuring tape.
As for a backpack, though there are lighter, I find the Granite Gear Vapor Ki really comfortable. But I make a little modification-- I use my bandanna to rig up the bottom so that there is no bottom panel. The frame of the Vapor Ki is not a your standard frame with metal stays; it is very flexible. I sort of weave the bandanna around the ice axe loops and under the back panel so that the back panel meets up almost directly with the outer walls in sort of a V shape, like some of those small hydration packs. This gives the pack more of a form-fitting curve that rides better and is less restrictive to the stride, once the pack is loaded up with some weight. In fact, I think the fact that I can do this is why I find it this pack more comfortable than others-- most framed packs would not allow this much flexibility of backpanel shape, and the Vapor Ki still has most benefits of a framed pack. It sounds confusing, but it is pretty easy, and if your wife gets the chance to try a Vapor Ki in person, maybe she will be able to see what I mean. I often see them for cheap online. (For even more comfort, I pack my sleeping bag or puffies loose on the bottom, for some extra cushioning from pack contents.)