How could kei be friends with the ex? What he did was unforgivable no matter the reason.

I don't think that they're friends hahahaha he just go there to make him suffer hahaha he just go to brag about nanase knowing that the ex didn't stop loving him, what he did was wrong, he knows it and he will regret it forever and Kei just like to rub salt on the wound bragging about their relationship

but he was comfortable with him enough to have chat with him and even went to see him...

Yes, but the ex doesn't enjoy his company, more like he support him because he doesn't want Kei telling to his partner that he had a reason for what he did, yes Kei is confortable bullying the ex it's like...
When you lose something and then you see another person with what you lost and he has the nerve to rub it in your face and also go to you to tell you how he's enjoying it while you have to resing yourself, or so I think ╮(￣▽￣)╭