First encounter:
Sure, at first, she's met with sort of a shocked reaction but this is due to having not seen him in years. Same reaction was from Naruo also. She tries to attack him after to down him.
Second encounter:
She tries to attack him but she can't because of course, he's someone she cares about.
Possible reaction:
She may end up being somewhat surprised when he comes to the battle, but I don't see her getting all goggly eyed over him. I think we're gonna get her take on it, and then she's gonna wonder what's going on with Naruto and how he feels about his return. He may apologize, but I don't see her reaction being even close to Karin's. Like people have stated before, Karin is her own character.

Agreed, but also the fact of the matter is that even though they reunited before the Land of Iron, the Land of Iron reunion was the first time that she confronted Sasuke in all his evilness. Before that Sasuke was just a rogue nin.
Though on a another note, I wonder if the reason Orochimaru or possibly Sasuke decided to go to the battlefield so that Madara would keep Hashirama busy. Then Oro could just take control of the other Kages again.