It could be that the fanfic writer is trying to combine the two universes in a rather dark way. In Fairy Tail, there are all sorts of magic and temptations. If a character from Naruto's world gets involved with someone from Fairy Tail, they might be easily cheated on. For example, if a Fairy Tail character has a magic ability to manipulate emotions, they could use it to deceive Naruto's character.
Naruto could also experience a period of self - reflection. He might wonder what he did wrong in the relationship, even if it wasn't his fault. This could lead to him trying to improve himself in ways that are not really necessary. On the other hand, it could also fuel his determination. He may decide that he won't let this setback break him and instead use it as motivation to become stronger, both in terms of his ninja skills and his emotional resilience.
There could be many interpretations in such a fanfiction. Maybe it shows Naruto's vulnerability in relationships. It might be a way to explore his character deeper by putting him in a difficult emotional situation.
Ino could be a very complex character in this fanfiction. It's possible that she cheats on Naruto because she has a fear of commitment. Naruto is a very passionate and all - in kind of person, and that might scare Ino. So, to avoid getting too deep into the relationship, she cheats. Or perhaps there's a power struggle at play. She might think that being with a different person would give her more influence or status in the ninja world. And there could be a hidden agenda that she's following, something that she can't achieve with Naruto but believes she can with someone else.
In such a fanfiction, the Fairy Tail guild might be thrown into chaos. They could initially be confused about this strange event from another world. Some members might see Naruto's sealing as a threat to their world's balance, while others might be more curious and try to find a way to break the seal.
I think Naruto would be heartbroken. He's always been so open and sincere in his feelings. When he discovers Ino has cheated on him, he might go on a bit of a self - discovery journey. He'd wonder what he did wrong, even if it wasn't his fault at all. He could also become a bit more cynical about love, which would be a big change for him. He might start to see the world in a different light and be more guarded in future relationships.
In the long run, Naruto might try to find a way to redeem those who cheated on him. Despite the pain, his kind heart would still hope for the best. He would likely try to understand their motives and see if there's a way to fix the broken relationships, while also being more cautious and less trusting in the future.
He might also become more reserved in future relationships. After being cheated on by Anko, Naruto could be more cautious when it comes to getting close to someone again, always having that fear in the back of his mind that he could be betrayed once more.
Naruto might be initially shocked and heartbroken. He is someone who values loyalty and friendship deeply, so being cheated on would be a huge blow to him.