I really think the key is to get a bike small/short/light enough that she feels comfortable on it. You don't need massive ground clearance unless you are going to be doing serious off-roading, which doesn't sound like what you want to do. You can lower some bikes fairly easily and inexpensively. Weight is a BIG factor for riding off pavement. The difference between riding my 1200 GS and my KTM 530 EXC, even on simple dirt roads, is pretty big! More power is nice, but it is not everything. Low end grunt is nice to have. The ability to run at least 65-70mph would be good, but is not an absolute necessity.
On my trip to Montana in July, there was a lady riding an older Suzuki DR200 I think. I can't recall the exact model. She had that thing LOADED!! She managed to show up at the campsite every evening. She wasn't tall at all. I think she might have been in her late 50's or early 60's. I don't think she did many of the "challenge" sections of the route, but she did all the rest and she was having a blast. Total route was just over 1800 miles in nine days.
There was another younger lady there (40 ish) riding a Honda CRF150!! She did ALL the challenge routes! Her boyfriend was riding an XR650R/L and was carrying the bulk of their luggage to keep her as light as possible. She wasn't tall either, but she did have that aggression factor. She was only able to top out around 60mph, so they would drop back on some of the more open and faster sections.
Something like a Suzuki DR200S (carb) or the VanVan 200 might be good (FI). There is also the Yamaha TW200 (carb). They are very popular and have achieved an almost cult status. The Yamaha XT-250 is also worth considering (FI). There are also the Honda CRF-230L (carb) and newer CRF-250L (FI).
Any of those would make good entry bikes. They are lightweight and not intimidating because of power. They can be lowered a few inches without much trouble or affect on handling. However, they will have soft suspension, so loading them up with tons of luggage will likely overwhelm the rear shock. A stiffer spring and thicker oil would help there. If she decides that kind of riding is not for her, then you can probably sell any of these without taking too large of a loss on them. This would be especially true if you were to buy a used bike, which are often already set up for dual sport riding.
I she decides she really likes DS riding and gains skill/confidence, then she might be interested in something like a Suzuki V-Strom 650, Suzuki DRZ400, Honda CBX500, or other similar bikes. The big thing with them will be more power, so you can run highway speeds easier, and better suspension/brakes. You'll also be able to carry more luggage. They will tend to be more comfy for longer rides as well. They will be heavier though. They can also be lowered.
On any of these bikes, tires make a big difference when you get off the pavement. There are "adventure" tires that are designed to be 50/50 tires, or they might be slightly biased toward either side of that. They are usually big block tread patterns. The idea is to help with grip off pavement, but still give handling and lifespan on pavement. Examples include Metzler Tourances, Michelin Anakees, Heidenau K60s, Shinko 705s, Continental TKC-70s, and Mitas (can't remember the model). Like everything in life though, there is a compromise involved. If you know you are going to be doing pavement and hard pack forest roads, then these adventure tires usually do well.
IF you plan to ride where there is a good chance of deep gravel, dirt, and sand (like Big Bend), or heaven forbid... MUD, then those the shortcomings of those tires will become immediately apparent! You will want full knobbies on the bikes for those conditions. You give up some pavement performance and they won't last as many miles, but they will make a BIG difference in handling. I live in Huntsville. If I go West of I-45, I find more hard pack dirt/gravel. If I go East, I find LOTS of sand. So I like to keep a set of TKC-80s on my 1200 GS. I have used Shinko 705s and Heidenau K60s on the bike as well. If I could not run the TKC 80s, I'd likely run the K60s, followed by the 705s. The K60s will do sand, but it is a lot more work than the TKC 80s! A popular knobby for the smaller DS bikes is the Dunlop D606. They have great dirt and street performance. I ran those exclusively on my KLR 650. On my KTM, I run the Pirelli MT21s or MT16s.