Naruto might initially be in shock. He's a person who values friendship highly, so being betrayed by Mirajane would be a huge blow. He'd probably first try to talk to her, to understand why she did it.
In this fanfiction scenario, perhaps Mirajane has her own dark secrets or hidden motives that lead to her betraying Naruto. She might be seeking power and sees Naruto as an obstacle. Or, it could be related to loyalty to her own guild or group. If there are conflicting interests between Naruto's world and her world, she might choose to side against Naruto to protect her own people.
In most Naruto fanfictions, Mirajane is from the Fairy Tail universe and Naruto is from the Naruto universe. They exist in different fictional worlds, so the idea of Mirajane 'cheating on' Naruto doesn't really make sense in the context of traditional fanfiction. However, if it's some kind of crossover fanfiction with a really strange and out - of - the - box plot, it would be very rare and not a common trope.
One possible fanfiction could be about Naruto visiting the Fairy Tail world and teaming up with Erza and Mirajane. They could go on a quest together to defeat a powerful dark wizard. Naruto's unique ninja skills would blend well with Erza's swordsmanship and Mirajane's takeover magic.
They could start by meeting in a strange, in - between world. Naruto is intrigued by Erza's armor and Mirajane's different forms. They soon realize they have a common goal, which is to find a way back to their own worlds. As they journey, they share stories of their past adventures, with Naruto talking about his battles in the Hidden Leaf Village and Erza and Mirajane sharing tales from Fairy Tail.
One possible fanfiction could be about Naruto ending up in the Fairy Tail world and teaming up with Erza and Mirajane. They could go on adventures together, with Naruto's unique ninja skills complementing the magic of Erza and Mirajane. For example, Naruto's shadow clone jutsu could be used in creative ways during battles against powerful foes in the Fairy Tail universe.
He could be betrayed by a false friend in the DxD world who spreads lies about him. For instance, telling others that Naruto has a hidden agenda to take over their realm.
In a Naruto - Fairy Tail - Mirajane fanfiction, it could start with Mirajane traveling to the Naruto universe. She might be on a quest to find a special kind of magic that she believes exists there. When she arrives, she meets Naruto. Naruto, being the friendly and curious person he is, offers to help her. They then explore the ninja world together, with Mirajane learning about ninjutsu and Naruto learning about Fairy Tail magic.
One interesting aspect of Naruto, Mirajane and Lisanna fanfiction could be the crossovers. Writers might create a story where Naruto's ninja world collides with the Fairy Tail universe of Mirajane and Lisanna. They could have unique adventures together, perhaps facing a common enemy that combines the threats from both their worlds. For example, a dark sorcerer - ninja hybrid villain.
There are quite a few. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Naruto Mirajane' and you'll find a variety of stories with different themes and plotlines.