If your friend wanted to say that only people who spoke proper English and wore belts on their trousers could rent cabins, they could legally do so.
Personally, I would rent my cabins from somebody else--friend or no--and I would tell them why and send them a copy of my invoice telling them how sad you were that they forced you to take your business elsewhere, when you would MUCH rather have put your hard-earned vacation money into a friend's pockets.
And anyway, you have to wonder what sort of "friend" would deny their business clients the fundamental human right to self defense? Is your friend going to assume ALL responsibility for the safety and security of people staying in these cabins?
Vote with your wallets, folks--even if you have to vote against a "friend"...