This is a popular request. It's discussed in the "Lae’Zel Ending Departure (Spoilers)" thread and the "I hope origin spawn Astarion can persuade Karlach to survive in the ending…" thread.
I find it strange, because when Wyll went with her in my game, he volunteered himself, without any prompting from me. My character chose the most neutral option available when talking to her in that scene, so it felt like her decision there too. It doesn't make sense to me that the player has to be present for Wyll to volunteer.
