Pokemon that can learn False Swipe and Odor Sleuth:
Pokemon that can learn False Swipe and Foresight:
Farfech'd, Sneasel, Weavile, Rowlett, Dartrix, Decidueye, Smergle
I would Recommend Farfech'd or Decidueye/Dartrix.
Decidueye can get Roost to heal, and can finish off some Ghost Types with Spirit Shackle/Leaf Blade. Dartrix isn't weak to Ghost, as well as doing everything Decidueye does so that is to consider. Then again, at level 100, that matters much less.
Farfech'd gets normal STAB, can use Roost for healing, has moves to finish the Pokemon off. Farfech'd is also immune to Ghost moves, so it may take less damage.
Smergle can be used as well, but has quite bad stats. If you do use it, make it's KOing move Pay Day so you can make some money while your at it. :P
I wish you luck in finding your sister a shiny Gengar! It is a very a kind gesture.
Hope I Helped!