I agree with NanKate teifi. You need a properly drawn up legal document to protect your financial interests. I know we don't like to think it's necessary when it involves our own AC, but sometimes it is.
We thought we had everything protected when we invested, along with my brother, the money our ES and his wife needed for their deposit but we hadn't been careful enough and now he's estranged us, we'll more than likely never get our money back.
Things change, things can go wrong so unless you can afford to lose what's needed to pay half off half his mortgage, please be very careful.