While it seems some management might be required to sufficiently route high traffic partners, does it really require 'building new pipes' or 'special infrastructure' that has a non-trivial cost?
I thought the wonderful aspect of the Internet is such tasks and distribution could be had for very, very cheap.
To fix this, you can try to talk to that any of the 3 parties involved, or you find a new peering agreement (if possible), or you actually start paying for the traffic.
Or you just leave it as-is, hoping that at some point somebody closer to the congestion starts complaining, which eventually is going to happen.
This certainly is the cheapest route and this is what we honestly can expect considering that all we are promised is 'best effort'. Everything beyond that is really cool to have, but also comes with some expense for the providers.