My suggestion would be to sell the plane or get one of you to sell the share to the other. Selling the 50% share would of course be the better option as you are willing to buy it.
Another option would be court or solicitor based mediation, and although no legal contract exists you have a verbal agreement and should for the sake of expenses be able to prove that this is the case, ie use bills and payments to demonstrate the 50/50 split.
The aircraft in the mean time should remain based where it has been as not to fundamentally alter the original agreement.
Also look at the questions such as who is driving the changes and why? If the skirt is driving the changes, unless she has a share now given as a gift then she has no say.
Is he using to teach her to fly? Do you agree with this if he is and do you have insurance which covers that? if not it could prove costly for both as the owner will be held accountable.
Just my thoughts.