Plumbing problem expert advice needed

A friend of mine called me last night to see if my handyman knowledge could fix his leaky pipe.

Here’s the problem. Under his floor are the 28mm copper pipes for the heating boiler. At one place there are 90 degree soldered elbows. The pipes run almost touching each other. At this bend the inside pipe elbow is leaking so bad that the water drains out of the system. So how do you fic that joint when all the pipes are so close and there is little flexibility in the pipework to replace anything?
Any ideas? Perhaps some sort of sealing putty?