Do you know what the £100 is made up from?
If he was renting a phone as in a handset, then BT will charge for that. If he has had it for a while, he may want to contact BT and ask where to return it. Chances are they'll say "keep it"
These rented phones come with a minimum term, so if he'd just rented a new one and was still in term, and it's the rest of the minimum term they charged him, not a lot he can do about it.
If they ceased the line totally (no dial tone at all from the phone) then they charge £124.99 for reconnection. You can get a deal that knocks it down if you sign with BT for 18 months... latest deal is here :-
http://www.serviceview.bt.com/list/Public/notifs/22-04-2008/Notice08_boo/NotificationPeriodImpl267 403FrameworkImpl267413.ht m
Hope that helps.