the only way i know to change details like this would be for someone to have called / written to them. In the past my details have turned up on utility bills because ;
1) the landlord has made the call (in rental accommodation) or 2) i have made the call.
Its worth noting that whoever asked for the details to be changed should also have been asked for meter readings .. again based on my experience.
Changing it back to your friends name should be as simple as calling eon and telling them the account details are wrong.
hope this is of a little help