On the other hand, Sasuke could be very stoic about it on the surface but internally be in turmoil. He might start to analyze his own actions and whether he had missed any signs from Sakura. He could also be the type to immediately start thinking of ways to win Sakura back if he realizes he can't stand the thought of her with someone else.
In a more complex scenario, Sasuke seeing Sakura kiss could be a turning point in their relationship within the fanfic's universe. If it's out of the blue for him, he could become angry or confused. He might question Sakura's loyalty or his own assumptions about their bond. It could also lead to him distancing himself from her, or conversely, finally realizing his true feelings and trying to win her back.
The man Sakura is with could be an undercover enemy. He might be using Sakura to get to Sasuke. Sakura is unaware of this and Sasuke, seeing them together, starts to suspect something fishy. He then has to figure out how to protect Sakura without revealing his true feelings for her.
Well, it depends on where Sasuke is in his character development. If he has become more mature and self - aware, he might approach the situation with more calmness. He could try to have a conversation with Sakura to understand what's going on. Maybe he'll find out that the man is just a colleague from the hospital where Sakura works, and they were discussing a patient. But if he's still in his more brooding and possessive phase, he might cause a bit of a scene, or at least give Sakura the cold shoulder until he figures out the whole situation.
Sasuke often appears in different story arcs of the Boruto manga, depending on the current plot and mission. Sometimes he's on a solo mission, sometimes he's involved in events related to the village.
Naruto is likely to be popular too. Even though it's a Boruto x Sasuke fanfic, Naruto's relationship with both of them is so important. He could play a role in supporting them from the sidelines or even getting involved in a crucial part of the story.