It happens after you talk to Nettie, in the room with the drow on the table, after you read Halsin's notes. Tav specifically says that Halsin looks like the best option.
And no, at this stage it's not reasonable to say that Halsin is the only option. By this point, Laezel has already made her pitch about a creche, and Sazza has already brought up Priestess Gut. The only way you can say Halsin is the only option at this point is if the player just skipped the other stuff.
Asuming you even talked to Sazza, or she is still alive ...
Same goes to Lae'zel ...
But even if ... and that alone is a big if, your other options are:
eiter a Goblin, creature that is known for being treacherous, canibalistic, sadistic, and few other stuff for wich most adventurers tolerate them only on the pointy end of their swords ...
or a Githyanki, creature that IF it is even known to your character, is known for being xenophobic, cruel, and existencionaly hating anything that is even in slightest way tied to mind flayers ... as that little creeper in your head for example.

So yes, basicaly Halsin is your only option.

Unless YOU are Githyanki yourself, obviously.
I claimed that Halsin is clearly an important option based on the plot as presented.
And i disagree with that claim ...
Since you can easily kill Halsin and basicaly nothing will change, you just wouldnt have one big elf in your camp ... and Icelyn will will frown on you.

Maybe i take this a little too seriously, over all those past conversations, when people were trying to persuate me about that: "Any other way than helping Halsin makes no narrative sense" ...
I completely missed that you call him being just an "option" this whole time. -_-
Several people have mentioned that they don't think they can win the fight without relying on exploits.
Yeah heard that before aswell ...
Funny that some of those people, if pushed long enough, admits things like they concidered Potion as a Bonus action to be an exploit. :-/
That should be the question here ... what "exploits" are we talking about?
I am saying that the fight is winnable without leaning on exploitive tactics. This is entirely relevant. I can't think of anything more relevant. I don't think the fight should change because I enjoy the challenge, and furthermore, the fight is winnable. If the fight were not winnable, I would agree that something might need changing.
Cant say i dont understand you ... i was in the same spot not so long ago.
Many people were trying to persuate me that fights in this game (and this one especialy) are quote "created around exploits and gimmics" ... and i just like you were arguing that since the fight is doable without either of those, i simply cant agree with such claim.
I also like the challenge, even tho i still believe i would like it much more if they werent cheating. (HP+stats, the same song over and over)

But it is irellevant, bcs if (and yes its just an example i dont have statistic, but Larian do) 90% of people gets massacred by Githyanki ...
No matter if *you* specificly managed to kill them or not, or if you enjoyed the fight or not ... what matters is that so many players didnt even stand a chance.

And yes, it obviously works both ways, wich would make your opinion part of that relevant group if the statistic would be in fact other way around, im aware.

Just counting people in this very topic ... we seem to be in minority. :-/