Have you set out your arguments in writing to him? If not do so soon.
No, I disagree.
The partner is also a friend.
I would be upset if a friend felt it necessary to write to me. As I suggetsed previously, in the first instance, the two of you should meet, perhaps with another pilot owner who can give an independent view. Hopefully you can resolve your differences at that meeting.
If not, only then might it be time to write.