After the last post that Thomas made, I went back and re-read the original post. Some things just don't add up to me. How does one take their vocal skill from that of a choked chicken, and an out of tune one at that, to sounding GREAT in a matter of weeks? The original post implies to me that this is a client that has no business calling themselves a singer and Thomas has an issue with how to tell them that. With all the dedication in the world, I've never heard a vocalist make such a vast improvement to their tone and pitch in such a short amount of time. Was the original description a little too harsh or are you wanting to make good on proving your point?
I do think it should have been mentioned from the beginning that the client is also a close friend of many years. This changes everything to me, and how you should have been able to approach the topic with them in the first place. I know I have different working relationships with different clients, some want me involved with production opinions and some don't. All of these relationships seem to evolve pretty naturally over time.
Finally, any ME that takes a client's music and applies their technical skill to ensure that the music will translate as well as possible, regardless of their subjective opinion of the performance, is providing value.
I can't help but wonder how many classic albums that have reached "iconic" status and may also be a bit "rough" around the edges would have been turned away by Thomas. If you're mastering an Aretha Franklin album one day and a Stooges album the next, is one less ready to be mastered than the other?