Did Tadakoro really forgive Kirigaya? She says "Thank you for your apology", which to me falls short of actually forgiving, let alone anything resembling friendship. It is more along the lines of "At least you realized you did something horrible. Live and let live, but don't ever do it again" (modulo nuances lost in translation, cultural filters, etc.). Tadakoro-san isn't the vengeful kind. What Nikaidou-san is capable of when something threatens her beloved Tadakoro on the other hand... well, we have seen some examples.
(Then again I hope the main bad guy in Tadakoro's manga turns out to look just like Kirigaya. It doesn't take much to turn those hair clips into the horns of an oni).