I guess 水中撈月 is right. There is misunderstanding here. I modified this post.
I did not suggest them 轉入地下 or 皇上麵前一套,自己低下一套.
I suggest them to use that coach as a personal coach. I do not think "Doing personal coaching = “私下”打黑工".
I know in many sports, a player can have his/her personal coach in addition to his club coach. A person can choose to be or not to be a personal coach; that is his choice.
Hopefully my explanation makes it clear.
I just propose suggestions to help.
thanks again, 水中撈月, for your understanding.