My friends often ask after my parents = My friends ask me the question, "How are your parents?". When I speak to my parents, I can say, "My friends asked after you." Use 'ask for' when you want something. "At the end of the meal, we asked for the bill." If you want to speak to a person, you ask for him/her. "I wasn't happy with the service so I asked for the manager."

To 'ask after' is really not used much any longer. We use 'ask about' a lot more.