DIY jacket cuff extenders?

You could sew on polyester stretch knit cuffs (think Stoic Hadron Down Jacket) to get another several inches. Should be an easy mod. Like these: https://www.joann.com/adult-knitted-cuffs-2-pkg/2301257.html

TBH, I know this isn’t answering your exact question, but my vote would be for her to return the jacket and keep searching until she finds one that fits properly. I know it can be difficult and frustrating, but the proper fit will look much better and keep her warmer.

Alternatively she could wear arm warmers or long gloves/mittens. But I don’t think there is really going to be an excellent solution.

yeah, probably better to return it if it’s new.  Find out what sleeve length you need and select another jacket with longer sleeve.

But, you could make a tube of fabric with a french seam.  sew it to sleeve of jacket through the french seam.

I’m not particularly long-limbed and I’m definitely not skinny and I can tell you that women’s clothing is NEVER long enough in the arms. Return it and buy a men’s jacket. If you really cannot return it, get some arm warmers and scrunch them up around your wrists. This will still not be as warm as if you had gotten a men’s jacket.

Google “arm gaiters.”

I too would opt for a jacket that fits. Other than gloves with a long gauntlet, I would find add-ons clumsy.

These might work:

https://www.moosejaw.com/product/houdini-power-wrist-gaiters_10437348

I have very long arms and I seldom get jackets or shirts with sleeves long enough [ or for that matter big enough in the chest] now that almost everything insulated is made in Asian sweatshops.
My solution was to sew a socktop to my LW down parka to give me an extension long enough to cover my wrists. The exception to this has been my heavy Expedition gear, that still seems to be cut properly nut that style of clothing is neither cheap or light weight

Continuing the thread drift, but it might be helpful if you decide to return the jacket.

European brands usually have a much narrower cut with way longer arms than American brands do. Especially Mammut comes to mind, if you stick to their mountaineering jackets. (Be careful, they produce more and more casual wear, just as TNF does since a long time.)
Yeti, produces high quality down jackets, which are cut slim with long arms, too.

btw, I do understand the problem you are talking about.