When you work in consulting the client is not always right! In fact, quite often they are dead wrong.
And this is the value of being in consulting. They are speaking to you because they are hoping that you have differing experiences, skills, understandings or perspectives that you can ring to the table to help them with their problem.
Generally they don't just want a nodding head . If they did they would open the door and invite in one of the people who work for them.
Telling a client that he is wrong shows a strong and resilient relationship, demonstrates your value and what you bring to the table, and if it is done correctly, it can place you safely in a position of trusted advisor .
A very powerful commercial position to be in...