Does she do many other activities that use different movements than swimming, a well balanced fitness plan is best.

Also any other repetitive movements like computer, gaming or texting??

I am wondering what her most bothersome symptoms are & avg pain level if any?
Types of TOS- arterial , venous , nerve , disputed ( no conclusive type or cause) , also a mix of any /all types and pec minor syndrome.

TOS is so different for each of us, and different causes and complications sometimes, there's no set action plan that fits for all.

There is a thread about intelliskin garments, how is her posture overall?
Head aligned with shoulders, knees, hips from the side view? Or forward head and shoulders, if so posture /bodywork is a good idea.

A posture brace may cause some to depend on it to do the work vs learning to be aware & fix posture on your own.