Another choice for reaching UM cheaply, only just launched, is the Finarea VoIP company
www.internetcalls.com - call charge 10.44c ?, about 12.5c US - the very interesting part is call forwarding and an incoming number, which means it's usable without an internet connection.
But both Vlad's idea and this diversion method mean you would provide a UK number and pay for the incoming call diversion yourself. Or does your friend use VoIP and could try setting up an account?
But note that these Finarea VoIP companies have free calls to plenty of destinations, but require an initial credit which does expire if unused; 120 days seems to be typical