Thanks for your reply.
I'm still not totally sure what to advise hime though, but I do see that there are potential problems with agreeing the issues you mentioned with his friend.
He is an unusually 'mature' 17 year-old at the moment, who has a steady job in the Leisure industry, although he doesn't have much in the way of savings.
On a more basic level, can you explain what 'capital is, and how much he would need? (I think they property they are looking at costs around £130,000)
For example, if he needs a deposit, does that have to be his own money, or can he borrow that? And if you had any idea what repayments might cost, that would be something I could tell him...
As you can perhaps tell... I really don't know where to start with the advice!