I'm not sure the lady will have to prove where the finger came from.  I think Wendy's is screwed unless THEY can show that the lady planted it.  Even if all their employees have their fingers a disgruntled employee could have put it there as a practical joke in the same way you hear stories of employees at Tacobell taking a dump in the beans.  Maybe one of the employees has a sibling in medical school who works on cadavers.
I'm not saying that the lady didn't plant it herself, I'm just saying that I doubt a jury will give Wendy's the benefit of the doubt and, in fact, I doubt a jury will hear the case.  Wendy's wants the public to doubt that the finger came from them, but will still settle with the diners.  That's how I think it's going down.