I don't know why she had her spontaneous pneumothorax, but the commonest cause is an enphysematous bullae associated with smoking and i guess that is the the cause of your daughters problem.
Some resolve without treatment, but usually shortness of breath and pain necessitates the insertion of a drain, to let out the air.
I don't know how long the pain will last, as it depends upon what is the cause. She will have a chest Xray on Tuesday and the result of that will depend upon the next step in her treatment.
Give her Ibuprofen 400mgms for the pain as often as is necessary up to 4 hourly.