If you have 3 members in your party after the fight on the roof you can recruit her into the party right there. If not and you have 4, she'll tell you she'd like to join you but can't since your party is full.
You either have to go all the way back out of Moonrise, ruining the suspense and the pacing to get rid of someone, or watch Jaheira 'do a Halsin' at exactly the part of the story that she's been talking about for the entirety of Act II.
Also we can't have Jaheira and Dark Justiciar Shadowheart together in the same run? Like we all better get into platinum blond, looking like Uncle Leeland I guess hehe. If you Durge Isobel and follow the beats as presented for Villainy, you lose both Jaheira and Misc in Act III, since she needs to be around for his recruitment quest. Nevermind that she was perfectly fine working with the Slayer in the last game to get the job done 100 years ago. She might as well be a Paladin of Selune here I guess, but even then, what happened to "travelling with you is never dull" ???
What's the point of making all these bad choices, if we can't be a disappointment to Jaheira and Minsc and Boo!?
In my view they're taking way too much off the table here to service some sort of perfect "party harmony" ideal. As players we only get these memos after the fact, which is frustrating.
I think the OP was talking about recruiting her at the entrance of Moonrise, where otehrwise she would run headfirst into the fight and risk getting killed. There you can recruit her even with a full party. Later on the rooftop, she is no longer in danger and you can leave her there.
And yeah, if you are evil then you have to bear the consequences.
