Does your friend have their birth certificate? (The full one, not a short form) If so, does it say anything on there about the birth record being amended? If not, you could have a look on the GRO website at the birth index. If it says something like "Occasional Copy" then it suggests that there may have been an amendment.
This situation must be so hard for your friend, and aside from trying to ascertain the truth, it might also be of benefit for your friend to receive some counselling/therapy in being able to process some of the emotions that they are feeling.