Well, at some point there is a line to draw, otherwise if we follow all the requests we will spend a lot of time and finish by building an AI that will play instead of the player ;)

There is a confirmation with the cost displayed before validating the action. It's up to the player to know what he/she can do at any moment in the game and to decide what is his/her best course of action.

Telling him/her that there is a better course of action that could be taken is a grey area, and some players may actually resent it. Have you never been irritated by this player at your table that always points to you a better action that you might take when you are playing? :D