They just had to go with the typical shady Yakuza/mafia type of scenario, where the girl belongs to a family that goes bankrupt, then is "saved" by the Yakuza/Mafia on condition that the girl becomes theirs.
In this scenario, Arisu became Shiba's girlfriend.
NTR: Having one's partner stolen by another.
Arisu is Shiba's girl, yet Kazu and Arisu were a thing, making Kazu the guy that stole the partner in this scenario.
Shiba found out about it, and now took Arisu, his partner, back from Kazu, the guy that stole his partner.
Victim: Shiba was apparently Arisu's boyfriend
Stolen Partner: Arisu was apparently Shiba's girlfriend, but got stolen by Kazu
Villain: Kazu apparently stole Arisu from Shiba
I hope Kazu is able to steal Arisu back from Shiba again soon rather than later!