Hooo boy here we go. While I can understand Akira's immediate reaction, this is how you let jealousy turn into a poison that burns you up from inside and makes you into a monster. If she takes breaking Mayuki and Rin's relationship as her primary goal, abusing her position as close friend to both, things can turn really ugly...
EDIT - To add, I don't get the people dismissing Akira outright. Anyone who's developed - from their point of view - an intense relationship with another and gets to hear they've lost, if they claim it didn't shake them from deep inside is either a saint or a liar.
EDIT2 - On further reflection there's another thing to consider. Akira's obviously fallen deep into Rin, but she's said she's not suited for love. So Akira's stayed put and said nothing. Now with the reveal, there's probably another feeling too trending up on her chart: that of betrayal. Which is a strong motivation to take some very regrettable actions. The continuation depends on how quickly she comes out of the dark depths.