BEN : Anakin was a good friend.
LUKE : I accept the truth you were once Anakin Skywalker, my father.
It's all made clear in the end.
Beside do we even need to name this pupil in the PT?
When Ben talks about a former pupil of his that turns to evil, the assumption will be made by some that he is talking about the guy who turned to evil in the PT. Therefore he must have been called Anakin (sit on the Skywalker bit for a few films) back then and adopted the title of Darth Vader.
Therefore he was the one who betrayed and murdered Luke's father, Sir Skywalker (the guy who isn't Anakin).
If you have to give the other guy a name, why not call him Luke?
That way people will assume Luke Skywalker is named after his father and not after the friend of his father who is later betrayed and murdered by him.