How does alcohol affect a person who has been diagnosed with aortic valve stenosis?

Depends. How much alcohol are we talking about? In low doses (1-2 drinks per day), it would not have much effect, but if someone is short of breath before, it might even improve that. The main effects of aortic stenosis are fainting spells, chest pain and heart failure, and alcohol is not particularly good, esp in higher doses (> 2 drinks/day) for any of those.
Variably. Moderate alcohol consumption does not normally significantly affect someone with aortic stenosis (as) and normal heart function. People with aortic stenosis are susceptible to problems if they become dehydrated. Alcohol, especially in the summer months, can exacerbate dehydration and cause problems with as patients. The caveats are moderate consumption and normal heart function (no heart failure).