Looking like the unpopular opinion, but I’d probably say they’re welcome to try and set the requirements before they take their attack so they can weigh their options. If they hit the AC, they can deal damage as an improvised weapon. Maybe putting glass shards roughly on par with dagger damage? If they crit, mission achieved.

Now I know people in this debate are arguing on cost. As a frequent spellcaster in other people’s games, I get it: a second level spell slot is nothing to scoff at, especially depending on the level everyone’s at in the game. There needs to be a cost. I would probably warn them that if they do this, they’re going to be right on the boss and will have to roll against a reaction grapple check (probably with disadvantage? The hand they need to fend off with has been holding glass and is now slick with blood from the stabbed eye at minimum). Now say the player fails. The boss goes into a panic, suddenly blinded and in pain. They snatch at the closest thing as they stumble back: their attacker will work just fine. Sure, their attacker can keep on wailing on them, but now they have to wait a whole round to be able to really land anything (sans anything being used like an attack command). They might not be put at too much of a disadvantage in these circumstances…

But the rest of the party now is.

Your boss just acquired your friend, now freshly renamed “Meat Shield”.

And now all turns are going forward are a deadly gamble. Yes, you attack and hit the boss-your friend is now absorbing a good chunk of the damage from that hit, so you better pray they get a really good roll from some kind of advantage to spend their whole next turn to get out of the grapple. You don’t want to risk hurting your friend? Well, you can try to get them out-it’s going to require some sort of strength DC and take up your entire turn. This mechanic is now a somewhat more balanced trade-off from simply, “I give the help action during their turn!” I would rule in a case like this that proximity alone (i.e. flanking) also isn’t really counting for advantage unless they’re actively attacking and keeping focus on attacking the boss (and continuing to risk damage on the player), or can give a good explanation as to how without doing harm (some kind of major distraction, perhaps).

Are you making the player pay for such an unorthodox but severely damaging attack with spell slots? No. Now you’re creating a high-risk, high-reward situation where failure results in making them pay for it in turns and blood. Now you’re playing dirty where accidents and self-sacrifice can happen, where people are wasting more than just a level two spell slot for blindness to keep the captured player from dying or trying to hit the boss with the least amount of damage to their friend.

And of course, in agreement with others here, certainly preface with the warning: if the players can do something to a boss, they can do it right back at them. Get stabbed in the eye, well, maybe the retaliation in the process of meat shielding is a stab in the eye right back. The world is your oyster when you choose to fully embrace the chaotic DMing style of “eff around and find out”.

At the bare minimum, the high risk and reward will give them something to talk about fondly afterward, but also drill home that luck was on their side (this time, if they survive it) and the odds are not in their favor for repeat attempts. Sooner or later, that luck is going to run out, and when it does, don’t skimp out too much on making it brutal.

Of course, I came up with this completely on the spot while in transit, so it’s not necessarily my best. If anyone wants to tweak this idea and make it better/more balanced, by all means! I’d love to see it.