Western - specific woman's undergarment name?

What year are we talking about?

You might try flannel "combinations"-- see here: https://www.fashion-era.com/drawers-pants-combinations-knickers-fashion.htm

Well, from the perspective of someone who knows a bit about historical fashion (without being by any stretch an expert):

1. She wouldn't need to wear a man's flannel shirt for warmth. There were perfectly suitable women's clothing options for cold weather. In cold weather, women wore: heavier petticoats (made of wool, sometimes padded), jackets or cloaks, extra wool stockings, and/or fur-lined caps or bonnets. I mean, maybe her thing is wearing men's clothing, but be aware that that was extremely rare even in the West, though it's a trope that's overused in media.

2. Does 47/19 F mean high of 47 and low of 19? Seems like a hell of a big temperature range, but that's really not all that cold. Folks in Minnesota and Wyoming and at the higher elevations would laugh that to scorn, frankly.

3. Layers: by 1873, not a heck of a lot would've changed in the basic layers from the 1860s (which I'm familiar with), though bustles had come into fashion. Our friend wouldn't be wearing a bustle on the frontier, unless she were going somewhere and wanted to look smart/presentable. But no matter the level of fashion, the basic layers would be the same, just as today a woman will wear panties and a bra (just assume that's the case...) with a business suit, an evening dress, or a t-shirt and jeans. So she would be wearing: stockings, split drawers, a chemise tucked into the drawers, a corset (yes, a corset), and her dress (or outer layer). Combinations hadn't really started to come in yet.

In order to get to our friend's shoulder, our friend's friend would need to move aside the outer layer (dress or shirt) and the chemise. Chemises might or might not have sleeves--most work-a-day versions had little sleeves, so you would have to push the neckline down and over the shoulder; chemises had wide necklines, so this wouldn't be difficult. As far as the dress: it would fasten up the front, most likely with buttons, so she might have to unbutton the top few buttons and push it aside to get at the shoulder. In that regard, it wouldn't be that different from a man's shirt that buttoned up the front.

4. All that being said, I could see her maybe borrowing a man's shirt and wearing that over her usual underthings and paired with a skirt. It would be a bit odd but believable in the make-do kind of way.

Hope some of that is helpful!