I have chronic neck and back problems from various injuries, so the challenge is familiar. Upright isn't always better than too low or stretched out. Sometimes a more forward lean may be more comfortable.
And comfort may vary according to my progress with physical therapy, or setbacks due to other issues. I feel more joint aches with sudden shifts in barometric pressure, usually downward ahead of storms. I used to laugh when my grandparents mom predicted the weather with their aching joints, but I ain't laughing now that it's happened to me.
I'm often tinkering with the fit of my road and hybrid bikes. Quill stems and seat posts with quick releases help make adjustments as needed during rest breaks on rides. With my bikes that have threadless stems I'm pretty much locked in for that ride. So on longer club rides on rougher rural roads I'll often take my steel bikes instead.
After selecting an appropriate frame her size, start with an adjustable stem. This will help identify a suitable stem length and height for a fixed stem, if she wishes to switch from the adjustable stem.
The next best solution is a quill stem. Easy to adjust the height as needed with a single tool. A couple of two-bolt or four-bolt quill stems, shorter and longer, would make it easier to swap between the two without needing to strip the handlebar of brakes, bar tape or grips, etc.
So far I've used only fixed stems on my bikes, occasionally swapping between shorter and longer, angled higher or lower. But I'm considering an adjustable stem for my main hybrid for errands, commutes and casual group rides. It currently has a Nitto albatross swept bar, conventionally oriented (not flipped) and is generally comfortable. But it never feels quite perfect. Part of the problem is that the top tube is a bit long for my preference despite being technically correct. The swept bar helps with that.
Another issue for me is helmet fit and weight. With age my C1 and C2 neck vertebrae are deteriorating so I'm more sensitive to weight on my noggin. I often use a video camera on my helmets and I've found that most mounts set the cameras too far forward, straining my neck. Instead I'll mount the camera as far back as possible, using blue tack removable gum to hold the camera, with a safety leash as a backup. So far, so good, no mount failures. Much more comfortable on my neck, although it's not as comfortable as not having the camera up there at all. But I prefer to have at least one camera aiming where I look. (I've used video cameras on most rides for more than 3 years and one of the few times I didn't was the day I was hit by a car last year. If I'd been running video the driver's infraction would have been clear -- she was looking down at her phone while turning left across my path while I had the green light -- and the insurance claim would have been settled much sooner. Instead I'm still waiting and there's no resolution in sight.)
It also helps me to do stretching and massage before rides. A month or so ago I got a long handled percussion massager. Looks like a portable vacuum cleaner, with two heads about the size of golf balls that work sort of like fast moving pistons rather than non-directional vibrators. It's the bee's knees. I dab some topical analgesic balm on the tips and use the massager to apply the stuff to my shoulder blades, back and neck. Really helps before and after rides. Heck, I'd take it on a ride if I could. If I'm ever forced to switch to a recumbent I'm gonna have a massager built into the seatback.