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.
In a fanfiction, Naruto might plan a big public proposal. He could gather all their friends and family in the Hidden Leaf Village. He would make a big speech about how Hinata was the one who saw him when others didn't, and her love changed him. Then, with everyone watching, he would pop the question.
One possibility is that Naruto would plan an elaborate and very public proposal. Since he is a bit of an attention - grabber, he might do it during a big festival in Konoha. He could gather all their friends and family, make a big speech about how Hinata has always been there for him, through his toughest times, and how he realizes she is the one he wants to spend his life with. Then, he would get down on one knee and present her with a ring.
Iruka sensei might also be involved. He has always been like a father figure to Naruto. He could give Naruto the confidence he needs and maybe even help him plan the location for the proposal, perhaps a place that has sentimental value for both Naruto and Hinata related to Naruto's early days at the academy.
Hinata would probably be extremely shy at first. She might blush furiously and be at a loss for words. After all, she has had a crush on Naruto for a long time, so it would be a dream come true for her.
She might immediately go to comfort him. Hinata has always cared deeply for Naruto, so seeing him cry would likely trigger her protective and empathetic side.
He could be deeply hurt. Naruto has a big heart, and Hinata has been an important person in his life. Her abandonment might make him question his self - worth. He might go into a bit of a slump, not being as enthusiastic as he usually is. But then, he's Naruto. He'd probably eventually pick himself up and try to figure out a way to either get her back or move on with his life in a positive way. He might also wonder if there's some greater danger or threat that he needs to deal with that caused Hinata to leave.
The fanfic could show Naruto's initial anger and confusion clearly. For example, having him raise his voice and ask Hinata how she could do this to him. Then, it could gradually show his sadness seeping in as he realizes the relationship might be over.
It could also be that Naruto has unspoken feelings for Hinata. So when Kiba steals from her, it's not just about the act of stealing, but also a sort of territorial thing. He feels the need to defend her honor, like a knight in shining armor.
Hinata might react with anger. She's a strong - willed character deep down. So she could confront the person who cheated on her head - on. She might yell and express her disappointment and hurt. This would show her growth from a shy girl to someone who stands up for herself.