CMS are limited to the UK. If you leave the UK, the courts gain jurisdiction over child maintenance and UK court orders can be enforced in most overseas courts. However, in most cases it is expensive and difficult for the claimant and can take years (and the payer can just leave that country and go to another, starting the whole process all over again).
There would be no immediate grounds for extradition and it would not be in the public interest to pursue it for non-payment of maintenance.
If your friend wants to be untouchable in a westernised country then his best bet is Japan, if he can hack learning the language. Costa Rica is difficult to enforce a court order in too, as is the UAE. Some US states are tricky too, especially if the potential recipient of maintenance committed adultery and that can be proven.
It might be worth him moving his pension too if he can. UK Courts can't touch overseas pensions.