Not really my issue so I'm not sure if this helps but I wear a button up long sleeve hiking shirt over a tee shirt which works inside or out and can be adjusted to suite a range of conditions. I like the Columbia Silver Ridge but REI and exOfficio also make them for both men and women. A number of YouTuber hiker women wear them also though they may just wear a sports bra rather than a tee shirt. Dixie wears a red one in many of her videos. I only wear a jacket if the weather or temperature calls for it. In cold weather I replace or add to the tee shirt with a base layer.
A light vest comes to mind. Fleece or wool would be my preference. There are lightweight wind vests aimed at running and cycling and I would lean to the most breathable models for that option.
Is her upper base layer a thermal top or something? I recommend finding a sharp, nice looking shirt, that you wouldn't mind wearing anywhere, that meets the wicking or warming features you want, and using that as your upper base layer. Also, it doesn't have to be a 'backpacking' shirt per se, it can be any type of shirt built for outdoor activity or sports, particularly sports.
For some reason my wife will only purchase the 'item' if the 'item' is sold for that particular purpose, i.e. it has to be advertised as a 'backpacking' shirt, when the same shirt sold else where is at a much lower cost.
My two-cents, good luck!
@Cliff do you think your wife would consider wearing a lightweight vest over her base layer? I like this as an option to provide both extra coverage and extra warmth...a thin vest like the Arc'teryx Atom, Patagonia Micro Puff or the Patagonia Better Sweater might work!