Naruto would be extremely hurt. He loves Hinata deeply and to find out she cheated with Kiba would be a huge blow to him. He might go into a state of shock and disbelief at first.
In a fanfic like this, it might start with Hinata feeling neglected by Naruto due to his busy Hokage duties. Kiba, being an old friend, is always around to offer support. But this is a very uncharacteristic plot as Hinata is deeply in love with Naruto in the original series. It would likely cause a lot of drama and conflict within the Konoha community.
Naruto would probably be shocked at first. He might not have expected Hinata to leave him for Kiba. He could be angry at himself for not noticing Hinata's unhappiness sooner.
Most fanfics with such a premise are not likely to have a positive message as cheating is an unethical behavior. However, some might use it as a starting point to explore complex relationships and redemption arcs, but that's rare.
Naruto would probably be extremely hurt. He has always considered Hinata a friend and perhaps more. Betrayal from her would be a huge blow to his trusting nature. He might initially be in denial, not wanting to believe that Hinata could do such a thing.
One reason could be for the sake of creating drama. Fanfic writers often like to explore extreme scenarios that deviate from the original story to generate intense emotions and reactions from readers.
Naruto's reaction would be complex. He would be devastated because his bond with Sasuke is so deep. However, he also trusts Sasuke to an extent. So, he would be torn between his immediate emotional response of anger and his long - standing faith in Sasuke. He might start to look for clues and evidence, and in the process, he could end up uncovering Kabuto's devious plan while also reaffirming his friendship with Sasuke. He would probably be more determined than ever to set things right and prove Sasuke's innocence if he is indeed innocent.
Hinata would be overjoyed. She has loved Naruto for a long time, so his proposal would be like a dream come true for her. She might start crying tears of happiness and immediately say yes without hesitation.
I'm not sure if there is a particularly popular 'hinata cheats on naruto with kiba fanfiction'. Fanfiction varies greatly in popularity and quality. Some might find such a plotline unappealing as it goes against the original relationships in the Naruto series. However, in the vast world of fanfiction, there could be some out there, but it's not something that is mainstream or widely known to be super popular.
He could also become very sad and withdrawn. Naruto has always valued his relationships, and losing Hinata's love would be a huge blow to him. He might start to question himself and wonder what he did wrong.
Naruto would be over the moon. He's always been a family - oriented person deep down. He'd probably start blabbing to all his friends about how amazing Hinata is and how excited he is to be a father.