Where to apply primer when repainting shingles?

Hi everyone,

My sister hired some guys to repaint her house. They didn’t prime 80% of the house, and immediately after they “wrapped up” my sister went outside and could literally flick a shingle and sheets of paint would fall off.

She’s having them redo it properly. She’s wondering if when they redo it, primer should be applied to the entirety of every shingle where any paint has come off, or if it should just be applied to the visible naked wood in the picture? As you can see, some of the paint stuck to the naked wood - not all of it is falling off in sheets. So for a shingle where like half of the paint was pulled off, but the other half has paint that’s adhered to the wood alright, should they prime the whole thing or just the naked wood?

Thanks!