Something that I'm surprised people haven't pointed out already.
The whole deal is based upon your friend's mother bailing him out if things don't work out financially. Have you considered that a few years down the line he:
a) might fall out with his mother = bad for you
b) if this go financially awry, his mother decides to bail out and not support him or you = bad for you
c) His mother dies, your friend dies or whatever and can't be part of the mortgage anymore? (morbid but things happen)
To be honest, having her "word" means absolutely nothing when one has to face up to thousands and thousands of pounds to pay. Imagine things go wrong (as they are likely to go at somepoint in life - we all go through rough patches) and his mother has to support her son in the tune of a lot of money, she will think the following "I can either pay this guy a truckload of money to support my son's 1-room accommodation in London or I can tell him to shove it, pay nothing and get my son to rent a room somewhere else for exactly the same amount of money he is paying right now, oh in the latter case I'll be stepping on my word..what to do, what to do?".
I think your friend's mother is taking you for a ride. The mortgage is effectively on you to support as your friend doesn't have any financial abilities to help if things go bad and his mother only gives her "word, promise, oath" or whatever you wanna call it.
All things aside that the others have pointed out which make your plan a horrible idea, I think you should also contemplate that too. When money comes between friends things can go bad very-very quickly.