Because malgus is the easy one to kill?
Malgus' shatterpoint would be his weakness in respect to Force Drain, she'd realise he's the only one she could best one to one, without Vader or Kun receiving serious injury. She'd help Malgus because she has to if she wants to win.
And it's listed as an ability, I think it's called Essence Transfer, or something along those lines. Maybe Vital transfer? Might be Vital transfer, I'd have to check and I can't be bothered waking people at the moment
Either way, the power is listed in the guidebook which details each individual power, as being the ability to transfer her life or Force essence into another being.
Just should note..... Vader has Shatter point as well.
Edit: also i put forth my final arguement already Aub so any time