How to structure this title

Unmarried couple and guy’s parent is thinking of buying.

How to structure the title in a way that the unmarried woman (who will not be bringing in 25% of downpayment and mortgage payment) gets 25% of the equity upon sale, but in a way that she cannot force the sell?

Basically we are only unmarried because of an external reason and due to that she missed out on over 1 million of appreciation in man’s home where they both live in and consider family home. Marriage will occur eventually but maybe after the home purchase.

This purchase is for the guy’s parents, who will provide some downpayment. The guy will pay the majority of the mortgage.

Both parents and guy wants to give the woman a fair share, however guy does not want the woman to force a sale and force parents to move out in their old age if anything were to happen.

How to best structure this title?