There's no such thing as "Fall Protection". The watch can't protect you from falling, it can only detect the fall and call emergency services. The feature is called "Fall Detection".
Now, depending on the exact model of iPhone will be the answer to your question.
There's a difference between an iPhone 6, and an iPhone 6s. They are 2 different models.
If she has a 6s, running the most current iOS version which is 13, then Apple Watch Series 4 and 5 running watchOS 5 or 6 are supported on her phone and feature Fall Detection.
If she has an iPhone 6, then only Apple Watch series 4 running watchOS 5 is supported. A series 4 running watchOS 6 or a series 5 Apple watch will not be supported on an iPhone 6.
With that said, and iPhone 6 or 6s and a current iPhone work for the most part essentially the same, so there should be no real learning curve, ans everything she does on her 6 or 6s will work pretty much the same way on an new iPhone.