What I would do, personally, is purchase the land and have my friend buy his half of it from me either for cash or over time. The land would be surveyed and divided, with all paperwork made up all legal and filed with the authorities. Or, if the friend did not want to own the land and I wanted to maintain control of all of it, so he can't sell his half to some stranger, I would work out a long term lease arrangement. There's a lot of information on the internet about how to buy, sell, and lease land.
If you're trying to make something like a mini-
intentional community, I can't recommend strongly
enough reading
Creating a Life Together by Diana Leafe Christian.
Don't do anything casually or on a handshake, not even with your closest friend or a family member. That is fraught with peril. I tried sharing land with a family member and it did not end well. We got stuck with a huge pile of trash to clean up, and a small building to move. Years later we still have this mess to deal with.