Assuming this to be the case (it's a defence that has been used in the past and on some occasions led to a perverting the course of justice case) then all you need to do is name the driver and supply the address.
I suspect the SCP will write to that address and ask them to confirm that they were the driver at the time. At which point I doubt it will go much further, although the worst that will happen is a £60 fine and 3 points on a proxy licence held at DVLA on his behalf.
They may ask you to confirm that the driver had insurance, copies of flight tickets or some other proof that your friend was in the county at the time, however you are quite within your rights to refuse to do anything else once you have named the driver.
Personally I would be on my way to
http://pepipoo.com/FAQ.htm and fill in the NIP wizard.
Cheers
Paul
edited to add, they will only be able to fine your friend and create the points on the proxy licence if they attend court in this country, I can't see them getting an extradition or even attempting it for a speeding offence.
Please note I'm not an expert in these matters my post is based on reading many cases on various forums.