When to grab Yoshimo?

Currently in Chapter 2 with my party of 5 custom characters. I've done everything there is to do in Chapter 2 and am moving to Chapter 3. I know you get a nice reward for Yoshimo's heart after you get out of the Underdark. When is the LATEST I can have him join my party for this to work? I've been trying to limit NPC's in my group unless I need them for quests, to maximize my 5 characters experience.
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and please add to the thread title the word SPOILER.
maybe they are few, but some first time players can ignore what happens to him and still read those forums to get a better knowledge about how to play it, but not on his plot, in my first not spoiled play through it was a big surprise.