Welcome
@kasey00!
Have her decide what she might like to be and start working on that persona and build on that. Heck some girls married or dressed as soldiers at 15 years old back then! LOL Check out videos on YouTube with civilians in them and the many various Civilian reenactor Facebook pages too. There are many roles she could portray as a civilian...just as many as there were females who lived during the civil war there were various roles for them. Farmgirl, nurse in training (read books or wrote letters for the recovering soldiers) etc. Does she have a special talent from the era she could share? *see pics at bottom
FYI- At 14 her clothing would have been identical to an adult females dress --- only smaller. There was a habit of dressing children like miniature adults. The rule wasn't written in stone -- 14 yrs is kind of half way between and dresses could go either way. A well behaved mature 14 yr old would be dressed identically to her mother. Where as a string of 4-5 sisters one yr apart in age where a 14 yr old attending to an adult function would dress as an adult ---- while playing games with her sisters at home in the yard she would dress with a slightly higher dress hem.
Authentic era teenage pics: https://www.vintag.es/2016/05/lovely-portraits-of-victorian-teenage.html
Simply wearing a smile, bonnet and carrying a water bucket, basket of flowers, fruit, bandages etc. would work very well. What soldier would turn down a lemon drop offered to him etc? LOL Or even a sepia photo of a soldier and say she's looking for her father or brother. Many a teenage girls were orphaned and became camp followers. She could easily be a Tillie Pierce persona with a different name! Plenty of gals helped the soldiers but because Tillie wrote a book she stands out. Just be sure to stick with natural fabrics like cotton, silk, wool etc. not only are they authentic she will remain cool on a hot summer day! Polyester types are don't breathe and her sweat won't evaporate and she'll become hot and "clammy" in no time. Plenty of things for sale on Ebay and Etsy if you don't sew ... including corsets and bloomers.

You could even find a local seamstress/tailor for a custom outfit down the road. She could just show up at an event dressed in era clothing and walk around and speak to everyone. She could also pick a historical figure, study her then portray her.
Trust me, as long as she looks "authentic" she should be welcomed...after all there is nothing wrong with just walking around being historical "eye candy" at first to help with the overall ambiance for the visitors. There are men out there who choose not to join a regiment and portray chroniclers or even snake oil salesmen!
Her character could change or evolve any way she wants it to with every event she attends! I was approached by a 17 year old at an event who "grilled" me on my clothes, corset etc. because she was chomping at the bit to come the next time dressed in era clothing! I was more than happy to help her with advice. I also had a young couple do the same at another event for the same reason!
I've been invited to join a local unit but thus far I just hang with them anyway because I have my own display. She'll meet plenty of people who would be willing to help her but in the event a civilian/soldier isn't real receptive just
ignore them and have her keep doing her thing...after all they're a "public" event.
But most of all tell her to
have fun bringing a brave female figure who suddenly found herself in the middle of a battle willing to help in any way back to life! Bravo Civil War reenactor/historian sisterhood!!!
I hope this helped.

Feel free to ask me any questions.
*The "daughter" in the video looks to be about 15 or so.
View attachment 260666
View attachment 260669