I think T-mobile will unlock your old phone for you. It shouldn’t cost as much as £30, probably more like a tenner or £20 at most. Probably a matter of her phoning them up and asking for the code.
A shop could do it, some of them charge ridiculous amounts though.
You can do it yourself – although be careful – there are utilities out there that claim to do codes for 6300 (and other Nokia BB5 phones), but don’t use a cable to connect to the phone. They are dodgy cons, as there’s no way to calculate the codes without access to either the phone, or Nokia’s database.
Don’t enter any codes onto it. You will brick your phone.
Joe