What he ought to do is take the portable stock-shackle things off, close his eyes, take off her blindfold, and tell her she can do the rest. Our boy here doesn't seem to want to rape her or enjoy seeing her like this. So he should aim for a show of trust, because anything less is going to be unsatisfactory and slowly build up that risk of her finding a chance to stone him. Such as being left unattended, even with her wrists restrained like that.
Like, seriously, stand up. Hold your arms straight down, wrists together; either palms facing each other or facing towards you, I can't tell which one is in the art (edit, on a second look, pretty sure it's palms facing towards you). Either way, it's uncomfortable, but doable. But note you can bend your elbows and lift your arms up; that makes the pose easier to manage, not less, so the board shouldn't get in the way. You can also lift at your shoulders and then bend your elbows. You now have your hands near your face. Now is the awkward part. Assuming you can't twist your wrists in the shackles, your hands aren't facing towards you, that's inconvenient. But you can stick out one or both index fingers. You should be able to reach back and dig one under the blindfold, then lift up and leverage it off.
Sadly I do not have one of those restraints and am very tall, rather than a small svelte gorgon, so I can't for sure verify it... But it seems really doable that there's at least some combination of bending your hands back and coming from a side angle that lets you get a finger in there even with the board extending out from your wrists. It wouldn't necessarily be comfortable, but totally doable.
Or, y'know, she could like, squirm a snake in there. That'd probably be easier, come to think of it.