What would his reaction be?
He'll be mad, same as what he would feel for Shikamaru, Ino, Choji and his other comrades. He would be a lot more mad if it was Sakura since he loves her or Kakashi who is like an uncle so to speak or Iruka who is like a big brother. The Nar+Hina shouldn't even still be going on, there's a ton more manga facts pointing to Nar+Saku. He told his dada Sakura was his girlfriend, she hits him but doesn't deny it
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. Then a couple chapters back Kakashi said Sakura's feelings about Sasuke aren't the same as they use to be (She use to love him as Kakashi remembers when she use too) and said she may like him but it's a different kind of feeling
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and he continues on the next page
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. Sakura only thinks it's her duty to save Sasuke.
Back on topic though. Naruto would be upset if anyone was transformed, would be even more for those he knows and care for.