My advice would be to tell your friend not to do it. It rarely works out well renting, or doing any sort of business, with friends or family.
I don't know what you mean by doing this 'privately'? I assume you mean not through an Agent? No problem with that. Dealing directly with the landlord, if your friend goes ahead with this, is usually better than having to go through an Agent.
Yes, as Chris say, in most cases the deposit needs to be protected. There are a few cases where it doesn't but they are few and far between these days. It would be worth asking upfront which scheme the landlord would be using.
Your friend should make sure he reads the terms of the AST thoroughly before signing. Personally, I wouldn't sign initially for more than 6 months (as a landlord or as a tenant).
But, as I said, be very wary of doing business with friends or family.