@Fibblet
I'm afraid I must echo the others. I don't think this is a good idea. You'd be pretty much tying yourself to him via this hotel. A few things to consider;
Would you (and this friend) really want to be stuck running his hotel for who knows how long if DP relapses waiting for him to get back on the wagon? Who knows how long it would take him to sober up? If he does. And how many relapses before you've had enough?
Are you and this friend actually contemplating putting money into this and putting your names on legal documents as co-owners? Because you don't 'buy out' unless you've put money in. And you can't force a sale for a business if you are not an owner. I mean, why would you have to 'buy him out' if you never put any money in and if your names aren't on the business? And in the event you're 'on the paperwork' what will you do if he refuses to accept a buy out? Even if you had a written agreement, it would still be an hellacious legal battle.
And finally, do you really want to be an hotel owner if you should succeed in buying him out? Because there is no guarantee that you'll be able to sell it on and get your money back, is there?
I'm sorry to be so harsh, but I am worried for you. And I'm afraid you may have those rose-coloured glasses on that we've all been guilty of wearing at one time or another. Every one of us (I daresay) wants to believe so much and so have made a decision based on what our addict swears mean they 'never to touch another drop'. And I also daresay that each of us has lived to regret it.
Remember you don't have to set yourself on fire to keep him warm. Your primary, actually your only, consideration should be to keep yourself safe and your finances intact.
If you want to work at this hotel as an employee, fine. Quitting is easy. But if you are contemplating actually investing or having your name(s) on it, please, please see a solicitor (without DP) so you can lay everything out, including his drinking, and find out what your chances would be of walking away unscathed should the shit hit the hotel fan and you've invested and/or your name is on the business. Because I guarantee that if he starts drinking again, he'll be the first one to walk away from it.
Again, sorry for being so harsh.