Telling the troll to eat her feet first so that the emergency portkey does not work?

Although this is often assumed, it has most likely not been the perpetrators real concern. CF:

Also a toe-ring with an emergency portkey to a safe location, in case someone kidnaps Mr. Longbottom and takes him outside Hogwarts's wards.

I strongly suspect it was to heighten the emotional impact on Mr. Potter, to be able to see her face.