Sylvanas

If he runs, the scary monster the girl turned into will eat him. If he stays here and she loses, the bugbear will eat him.

Fido watches as Magdalene leaves, and watches the two beasties fight. He's lost, totally and utterly. He needs the girl to get him out of here. If he runs, and she can't find him, he's helpless and alone until he's eaten by the next beasty, or slips and falls through the trees. There's no ground, so he's practically guaranteed to be injured when he stops falling. If he runs and she does find him, she'll eat him. And hopefully feel bad about it, but that won't help him. If she loses, he can try to get away from here whilst the bugbear eats her corpse, see if he can find his own way out. Or get eaten by something else.

Best bet, therefore, is staying put. He watches the fight, knowing that the victor will pretty much decide his fate, if he gets out of here or is eaten.