If to consult is to provide advice, then that advice might, perhaps should, include what the customer needs to hear, but does not want to hear. A fine line. How to walk it? To withhold that advice could mean to underserve, or even damage, the customer. To provide that advice, however, could damage the relationship.
If true friends are those who tell you what you need to hear, but surely you do not want to hear, at the risk of damaging or destroying the friendship, aren’t true suppliers-vendors-consultants those who do the same?