Bicuspid aortic valve is a genetic disorder which affects some people in selected families and is actually though to be more of a connective tissue disorder which manifest not only in the diseased aortic valve but also in the proximal aorta. Your daughter, although only having mild aortic regurgitation at rest already has signs of valvular dysfunction as well as aortic root dilatation. Neither of these values ( mild aortic insuficiency and mild aortic root dilatation,) are in themselves in the absence of symptoms suficient reason at the present time to support open heart surgery. However, progression of the aortic root dilatation and insuficiency has been show to be decreased by the use of enalapril, especially if hypertension is present. I am not sure what the test performed were, but if thery included a stress test to see how the aortic valve behaved with stress, it may have shown worsening of the aortic insuficiency with stress and hencre his recommendation for her to stop high endurance activities such as cross country or basketball. He is probably simply attempting to prevent aortic root dilatation and decrease the risk of aortic dissection, which is a life threatening complication of long standing aorti aneurysms.
(Thanks, pbanders, for your valuable posts on valve issues.)