This may have been answered elsewhere, or it may just be as straightforward as it says on the card, but..

My friend and I had a game last weekend and I was flying a Tie/D with Ion cannon, I shot at a ship with my Ion cannon and hit it for its last hull point.

The title says that I can perform a primary weapon attack after the attack with the equipped cannon, I assumed I could shoot again at a different target, however my friend said the attack must be against the same target.

Who is correct?

You are. Unless the card specifies a target, you get to choose.

Yes you are correct. I am probably wrong about this but I think the only time you have to keep the same target for a double tap is when the card says "Perform this attack twice".

Unless it specifies a target, "perform an attack" is any target. xD only specifies primary weapon, so any target is free game