Then 40 is over 20 years off. I'm not sure what that prognosis means, but it may be that the doctor thinks the condition will likely become bad enough to justify the risk of open heart surgery. Of course, that amount of time is enough to hope for new discoveries and methods.
There are at least two factors driving decisions to take invasive actions: quality of life and risk. Thus, if one can "live with" a condition it may be recommended that they live with the condition until the quality of life is deteriorates to an unacceptable point, then a risk is worth taking.
I believe the aortic valve problems can limit the amount of physical activity one can participate in. Seems that's what you said is one of the QOL impacts.
He just turned 18. He is, as of I know, under restricted activity. He is not allowed to participate in very strenuous activity and he tries to avoid very fatty foods. I'm sure that he being very tin is not good for him as well as he would if he would be very large.
You didn't say how old your friend is, just that surgery may be need at age 40. Is that age far off?
As I understand it,bicuspid aortic valve disease is slow progressing and that progression my be held back through rigorous healthy heart practices (exercise and diet). I do not believe there are any medications that will reverse the condition.