If you’re not First Aid Certified, refer them to a professional (read: doctor) and if you are then only recommend what’s within the parameters of your training (Red Cross says cold therapy for the first 24 hours). NSAID pain relievers (ie ibuprofin) also reduce inflammation but make sure they, again, get the advice of a doctor before ingesting any medicines. Rest is key so be sure she’s avoiding/minimizing upper body activities til it feels good (no pain at rest OR during activity). I’d be particularly wary in this case as any chest discomfort could be a sign of another, more serious condition that’s beyond a trainer’s ability to improve.
General consensus from my certifications is that ice is best for the first 24 hours to reduce inflammation & then it’s heat after that to improve circulation to the area. There are some views that vary from this norm so I’m curious to see what the others say. Some professionals I know recommend alternating heat & cold therapies in short succession after the first day, and one trainer I know that’s also a naturopathic doctor does an eastern tradition of heat therapy all the way.