I know someone with aortic stenosis. is it a very bad thing?

Depends: It depends on the cause and severity. Many older persons may have some degree of aortic stenosis, which is a narrowing of the orifice of the aortic valve effectively limiting the blood flow the heart can supply to the body and making the heart work harder. For many, it is mild to moderate and may never cause them any issue in their lifetimes. For others it may progress to requiring a valve replace.

Variable: It can vary anywhere from mild to critical. What did your friend's doctor say?