Sorry, but seat height etc. really won't help her very much. She should rest and take antiinflammatories (Advil, etc.) if okayed by physician. It is usually short lived problem, here is a portion of an article from Web MD.
A Short History and Some Interesting Facts about Osgood-Schlatter Disease
This disease was named after the two physicians who defined it in 1903, Dr. Robert Osgood and Dr. Carl Schlatter. Recently, a Finnish study found that 13% of the teenagers of Finland had symptoms of Osgood-Schlatter. Today there are over 25 million children in the United States of America, and over 350 million worldwide, who are in the typical 8-16 year old age group that is most susceptible to Osgood-Schlatter. The condition is caused by the powerful thigh muscles pulling on the attachment point of the patellar tendon on the tibial tuberosity (see below) during activities such as soccer, basketball, track, wrestling, hockey, baseball, gymnastics and other sports. It is also a common problem for ballet dancers, as well as participants in Highland, Irish and other athletic dancing styles. Originally Osgood-Schlatter was reported as being more common in boys. However with increasing participation in sports by many young ladies, it is becoming almost equally as common in them. In fact, perhaps because of the more delicate skeletal structure in girls, they could be more susceptible. They have a smaller tendon attachment point, with greater tension per mm². It can even appear spontaneously during a child’s “growth spurt”, and has often been called “growing pains”.
If you go to the web Md web site, and put in Osgood-Slatter's it includes a picture of the knee, and it's anatomy, and how the "patella tendon" is becoming strained at it's insertion. Let her rest until symptoms abate.
2005 Trek Madone SL 5.2 Project One, Lightning
Koobi Enduro, Stella Azzurra Tirreno ergonomic bars
2004 Raleigh C30 back to stock (thanks to my son!)
2002 ALMS TT 412 Hp@crank
"I thought of it while riding my bike" Einstein on the Theory of Relativity