So I just thought of when and how the two girls would've talked about this, and I figured it this way... Enjoy!
''You're not going anywhere, Astrid.'' said Heather as she entered the cell in which Astrid was unlocking Stormfly's chains around the dragon's foot.
Astrid made a quick gesture, turning around the moment she heard her new enemy's voice, taking hold of the closest weapon to her, which turned out to be a long thin spear, which looked like it'll break the instant it would meet Heather's axe.
''Maybe not.'' said Astrid threateningly. ''But I am going to enjoy this.'' She advanced the weapon, the sharp point of the blade at its end, the very tip shinning as it caught a beam of sunlight entering through the cracks of the wooden planks of the boat.
''I doubt that.'' said Heather, the grin growing on her lips as she swung her axe 'open' and closed the cell's door, heavy metal colliding against heavy metal. The noise rang all around the large under deck of the vessel.
Astrid took a good hold on the spear and with a war cry, ran for Heather, and by luck, the black-haired green-eyed girl dodged out of the way just in time to avoid the spear's end plunging in her stomach.
''You have no idea who you're allying with!'' Astrid cried out, turning on her heels as she faced Heather once more. ''After all we did for you!''
Heather frowned and attacked at her turn, acting like she really did want to fight Astrid, only missing her on purpose.
''You've never known what it's like to be truly alone, Astrid!'' she yelled back, holding her ground once more after Astrid ducked her blow.
Astrid was really having it with this girl. She had earned the riders trust, twice, and then betrayed them all, twice as well. Astrid yelled in fury and ran for her again, her spear above her head, only to miss her once more, the spear's blade plunging itself in the ship's wall behind her.
''Heather, you have to believe me!'' she tried again, making her realize she was on the wrong side. She tried to pull her spear out of the thick wood, but it simply refused to move.
Guards suddenly passed by, not seeming to have noticed them just a bit, and went rushing to get the other Dragon Riders to the deck where their commander Ryker had called them to.
There was a moment where Heather said nothing, standing at the other end of the cell close to where Stormfly was. Astrid was beginning to think she was considering it, but when she came to speak again, Heather had already cut her.
''Of course I believe you!'' Heather suddenly whispered as loud as she dared, being as quiet as possible. She lowered her axe and approached Astrid like nothing had happened. ''I'm so sorry about that, Astrid.''
Astrid looked shock and confused, but mostly shocked. For a while, she just stared at her, blue eyes wide, hands still trying to pull out her spear.
''Wait – you're not – this was all a ruse?'' Astrid asked, totally lost with this crazy story. Heather gave her an apologising smile and came closer just like on Dragon's Edge when they practised axe throwing in the forest together. ''It was a ruse!'' Astrid sighed in relief.
The two embraced each other tightly like it had been years they hadn't seen each other. They both sighed in relief the other was all right and broke the hug and looked in each other's eyes.
''I needed the Hunters to think I sided with them.'' said Heather as she reached out the cell door's key from her belt. ''I'm so sorry.''
''Don't apologize.'' Astrid told her as they both approached the door. ''You were fantastic! ''
Heather chuckled lightly, not wanting her cover all blown in the beginning.
''I believed every word,'' Astrid continued on, ''I was absolutely crushed. If you weren't telling me this now, you would've had your head taken off.''
''Now, I need to bring you back on deck where Ryker ordered.'' said Heather, kind of sorry for acting this way around her greatest friend. ''But I'll convince him to keep you up on deck so you can get to your dragons easily when you have a chance. But you can't tell any of the other riders. I trust you on keeping this secret, but the others... well, you know how they are. Especially Hiccup.''
''Couldn't agree more about that.'' she agreed and Heather unlocked the door and flew it open.
''After you,'' she said kindly before returning to that evil act. ''Astrid.''