Here is a thought on Bach and all the others that didn't give directions on how to play: Maybe these composers wanted many different varieties to be played, leaving a part of the 'final' art to those that are playing the piece? And/Or maybe these composers wanted to give a more in-depth task to the conductor, something more to do than making sure everybody is playing their part in the way intended by the composer? I think I shared with you this Canto Osinato piece before. The composer of that one intentionally (we know this since this was voiced) created a composition that left the way of playing to those who plays and conducts. If that is the case for Bach, I don't know. But if Bach intended for his compositions to be interpreted in different ways, the result is that there are infinite ways of playing. Some may be good, some not so good and some may be outstanding. I guess it'll be a forever search for the 'best' performance. Personally, I would love it when Bach intended all of this. It keeps the performers busy for an infinite amount of time to try and seek yet another interpretation. From a marketing perspective this may be a great way as well, since when allowed to leave the way of performing a piece in the hands of the performers (not prescribing what they shall do), performers may favourite these pieces over those that are completely described how to play them.
BTW, I truly believe one can give advice to those who one considers to be on a higher level of some craft. In the Dutch language, we have this saying: "De best stuurlui staan aan wal". It says something to the lines of: " The best captains are at the shore". This saying tries to state: Those who don't know the craft shall not try and direct others who know the craft. Well, that may be true for some. But I have the believe this saying is not correct, at least not for everyone and not in all situations. I'm convinced you'll be able to help your friend, and I'm even more convinced your friend will be super happy with your reflection about his work :)