My lawyer explained contract strategies to me very simply. It's like a marriage, it starts with flowers and chocolates and ends in the divorce court with much bitterness.
It is therefore wise to contemplate the end of the arrangement before the flowers and chocolate phase begins. It's too late once the marriage is made.
Another lawyer who worked for me taught me that it is equally important to spell out rights and responsibilities once the contract is dissolved, for example, who owns what liabilities for maintenance costs and suchlike.
If the aircraft is "worth $200,000" at the time old mate only paid 50,000 for a half share, then as far as I am concerned he gets his 50,000 back unless other arrangements are in the contract.