Nao friend is just being a nice guys and i think a bit of side story doesnt hurt if it were only focus on the main couple this would end alot faster
There's a difference to being nice, and to being a pushover. The mere fact that she raped his friend but he not only overlooks it, but never has her take accountability for her actions, nor ever consults Nao for such a traumatizing matter, shows how much he falls in the latter rather than the former. And not to forget how much she "depends" on him and takes advantage of his kindness and time, while being very clear that he's only there to either vent or to help her with her problems without giving him any agency, which is all behavior he allows but also in some ways, supports. There's a codependency that is very toxic but not being addressed, that reinforces how much Nao's friend is being treated as a doormat to Ikuno.
It also clearly shows the extent of his friendship towards Nao is because he's clearly prioritizing his "relationship" towards Ikuno rather than his friendship towards Nao, especially with how much he does behind his back. He's not someone I would like to continue being friends with since he's clearly hoe before bros.