Well, probably. If he can get it out.
He'd been holding the stupid ceremonial knife when he'd touched the cursed books, and he'd still been holding it when he got zapped to wherever the heck he currently is, and because his stupid outfit didn't come with anything useful, like a sheath, Dean had stuck the knife in his boot once he'd survived climbing down the tower wall. Whichever bandit had searched him hadn't been very thorough, as the hilt of the knife makes a pretty obvious bulge partway up his calf.
In his regular boots, getting it out wouldn't be a problem. These knee-high monstrosities, though, seem to be a variation on the Chinese finger trap, and each inch takes a small and painful eternity; however dull it had been out in the real world, in this one the knife has a definite edge, and it's leaving a thin line of fire up Dean's leg as he coaxes it along by feel, eyes fixed on his guard. It's starting to get dark, and the man seems to be slowly listing away from Dean.
When the hilt finally slides loose, Dean allows himself a very quiet hah of triumph before he clamps the knife between his knees to start sawing at the rope around his wrists. The horse noses him in the shoulder briefly, but Dean's guard has slumped over entirely by this point and doesn't so much as twitch.
Dean manages to slice his wrists on the blade a couple of times, but the rope gives more easily than he'd expected, and after another minute his ankles are free as well. He's cautiously--and awkwardly, since it feels like all his joints have seized up--hauled himself to his feet, still watching the sleeping guard like a hawk, when Benny says from behind him, "Well now, looks like you don't need my help after all."