Never in my life have I wished so much to break the fourth wall in reverse and bitch-slap that bitch.
Now, if it were me:
I would've gone to that staff room to 'confess' that I was blackmailing Reina on behalf of Manami and now have a guilty conscience after Manami took it too far. Turn the situation around and paint Reina as the victim (she is) and Manami as the main aggressor. (she broke the peace and tried to ruin Reina's life for selfish reasons)
Also, my response to her confession would be:
"I may be a piece of shit, but you are an even bigger piece of shit than me, and I like the good girl type - which you've just proven you're not."
Might as well rub some salt into the wounds and make her feel like she just sabotaged her chances.
I honestly find myself frustrated with the direction the author took this. My desire for instant karma has been left unsatisfied.