I think it would help a lot, if both parties recognised that the circumstances that led to the original agreement have fundamentally changed, and that the original setup is no longer workable.
At that point the obviously solution is to sell the aircraft to a third party at market value.
That is a basic and fair starting point. Only when both parties recognise that fundamential point, and accept that things will never be as they were before, can they start to look at other solutions to see if they are workable. Perhaps one can be found that is fair to both people, or perhaps not.
Expecting things to continue 'like they were before' is nothing more than wishful thinking, and likely to result in the loss of both a friend, and a lot of money.
dp