Well it sounds very familiar , but the TOS name doesn't need to be pinned on it yet as church person mentioned.
Might be best to not pin a name on it if you don't need to.. at her young age..

She was perfectly fine before this started, so I believe she can get back very close to before this happened.
It may take a year or more of very good PT , self care & chiropractic.
Maybe not that long since she is young...
It took me 2.5 years for me, but work comp/employer delays /rules etc and I was in my late 30s.

If the PTs are focusing on mainly on strengthening and generic stretching /wall slides etc..at this point , I disagree with that approach for TOS or TOS like conditions.

I suggest locating some expert multi modality /multi skilled chiropractors.

There are tons of videos on youtube about TOS and pec minor, body work..explore those for ideas too.
Alexander technique, is good for posture balance vs forced posture fixes.

Our sticky threads here have soo much info saved in them..explore those.

Slow her down on any repetitive use & poor posture esp upper body postures.
Stop aggravating activities..

Sharon Butler has a very good website & her book is in most libraries.. I found her information of fascia very fitting for my injuries. www.selfcare4rsi.com
I did use her very slow gentle stretches for a long time , but still needed chiro & expert PT for the final 'fix" so to speak.. I do still have some myofascial discomfort at times, but I am 50s+ now.