matty,
thanks for the post.
A lot of what happens depends on the course of her illness. If the physicians think that her valve irregularities are from carditis and rheumatic fever, often the antibiotic prophylaxis is longer then age 21, so I'm a little unsure of the exact course o fher illness. It is very important that the interpretation of the echocardiogram in patients with mild carditis from rheumatic fever is interpreted by a physician familiar with the mild forms of the disease.
Overall, if her symptoms resolve, and the amount of heart damage is minimal or nonexistant, then she will probably live a full healthy life. It would be important that with any symtpoms such as increasing shortness of breath she be evaluated.
good luck