I think there are always many options in these cases. I would still move through MFPA Book B since it builds on so many concepts and skills introduced in Book A with the same friends and terminology, but move faster if the student can handle more pieces. You could add the Primer and start from the beginning (review is ALWAYS good!), but assign "On Your Own Pieces" and see how well your student can learn independently. The Primer Sightreading Book is also an option as a supplement.

I often want to give students many DIFFERENT reading experiences as well as give them more pieces that use the entire keyboard, so I often use 2-3 method series at the same time. Of course you have to think in terms of teaching concepts, and then find the pieces that reinforce those concepts - this means you know each series inside and out. Music Tree, Celebrate Piano, and Piano Safari offer a nice contrast to Piano Adventures. Hal Leonard's Piano Lessons/CD packet for Book 1 has some nice improvisation tracks, and will introduce some new things like the quarter rest in the pre-reading section, which is a great opportunity to work on LIFTING the wrist.