If my friend is smoking on the porch, it will immediately be scattered by outside air, whether there is a breeze or not? will it harm me if i'm near?

Depends : If this is a singular event then it's not likely to cause any long term issues. But if this is happening everyday then you may get enough second hand smoke(if you are outside with your friend) to create long term lung problems. Just stay inside when your friend is smoking and don't let them smoke inside.

Smoking does harm: Hi. Your friend has an addiction (or a death wish) that will ultimately harm him/her if he/she doesn't quit smoking. Second hand smoke does cause harm; that is well established. The best advice is to stay away from smokers when they're smoking. Tobacco smoke harms everyone in contact with it. In a stiff breeze, if you're upwind, fine, but in still air on the porch, you're breathing that toxic fume