i must agree with what is being said above.
friends are friends and good for a cup of coffee (and some!!) but business is business and if they are real friends they will undrestand and not expect a free dinner.
maybe you could make yourself a few basic rules. you will answer friends their questions or even help them with work so long as it is less than 5 mins a week, beyond that they must pay, although as has already been mentioned, you can give them a discount.