He doesn't stop him. He can't really think of any reason to - nor any reason that he'd have a right to. Megatron has made his opinion on the matter clear, and any persistence on the issue would be presumptuous and rude. He's done all that he can do. It's not in his hands anymore.

That doesn't make him feel better.

He can picture some imaginary outcome where things become alright somehow, but it doesn't feel like much more than a fantasy, and a foolish, immature one at that. Time has changed things, and there's no going back. He didn't know why he thought there might be.

So he lets Megatron walk away, following him with his optics until he's far enough away that some of the tension can finally leave his frame and he isn't afraid of being overheard. What does he do now? He isn't fully sure.

"Goodbye, old friend," he says softly, and that's it.