So, why not borrow half the cash from your mate, buy the aeroplane outright then learn to fly in it. Once you are finished you sell the aeroplane to him, he learns then finally, after he is qualified he sells half of it back to you? You both get to learn on it legally as sole owners but then end up in joint ownership.
You need to trust each other but, to be honest, if you don't have that level of trust the I'd think again about joint ownership of an aeroplane.
good luck
3 Point