Surely if he has written to them to no avail then another address is
useless unless you tell us here what address he used.  The legal option
seems worthwhile.

When did the contract minimum term end, when did he cancel his contract
and what was his monthly billing date?  Has he been roaming recently
before the contract was ended?

Certainly with Orange, I received a cheque automatically from them the
billing date following the last bill.  Unless your friend has waited
that long, it's probably a bit premature to be chasing things up.  With
regard to the roaming issue, roaming charges can take a while to appear
on a bill I understand, and that may explain if they're holding onto an
amount to cover those charges.

D