Good advice from several Gransnetters above, and I would add that, despite your very best endeavours, you are probably not helping the young man to face his difficulties, which he needs to do if things are to improve. We don't know what the problems are, but as with many problems experienced by people with maladaptive personalities, he will probably have to hit the bottom before he gets help. Let's hope he does ask for help from the relevant quarters. Sadly, some people cannot be helped. None of this is your fault. You have done your best to honour your friend's wishes. Take a deep breath and walk away with your head held high.
To think that London, or anywhere else for that matter, does not belong to any one demographic