As she is 14 years old and having Rheumatic fever
,she can take 325mg ,3 times a day for 8 days.
It is not an Antibiotic.It is a Salicylate drug.
The recommended adult dose in Rheumatic fever is 80mg/kg/day in four equal doses.In children,it is 90mg/kg/day in equal divided doses every 4-6 hrs.
But,you should consult your doctor regularly and adjust the dose
Wish her speedy recovery