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Does she still play tennis regularly? Does she still play for extended time regularly?

Hydration is crucial if she's playing for extended time. Make sure she keeps hydrated for at least 12 hours prior to sports activities.

Do NOT wear cotton. Cotton is very abrasive. Find appopriate sports attire that may help reduce skin irritations. They are generally expensive but worth the money if you find the right ones.

Keep the area as dry as possible. Wash off excessive perspiration as soon as possible.

I'm not a doctor.

Maybe see another doctor if the previous one didn't help.