Looks like you have a lot of advice around the legal side with your friend but I have to agree with Slug. Interest rates are rising across the world, The US are still raising, the Euro Zone have indicated raising rates. Japan (who have had a 0% policy for years now) is also making noises that interest rates are rising. This will mean that with inflation also rising here in the UK the interest rates will inevitably rise here. The net result of this is that the people will not be able to pay the prices asked for housing as the mortgage payments will be that much higher, people who have maxed out their savings will be paying more, as house prices level off people will not be able to borrow more money against their houses and eventually house prices will drop, consumer spending will take a hit and then we are in a full blown recession. This will happen, the only question is when.
You said yourself it is too expensive for you to buy on your own, you are not the only one in this situation, if this continues, who is going to replace the first time buyer, at the moment it is the buy to let investor who can pay these "entry" prices, however as interest rates rise it will not be economical for them either.
I'll get off my soap box now, read all of the information and make your own decision, this is not meant to be a scare story, some of what I have said is my opinion, some of it is fact - at the very least make sure you and your friend can pay the loan back if interest rates rise in the future or fix it for as long as you can (well your friend can, it looks like you will be applying for a loan in a few months/years time - the rates will be higher then)