Dignities in Vargas
determining the temporarily friendship is a spatial thing that needs a 3D-structure as foundation. This is given in the rasi-chart only as this is as the planets were placed in the sky at a special moment (at least kind of, don't let's splitting hair here).
Divisional charts are no real charts at all (unfortunately the way we are plotting them leads to this confusion). They are a mere collection of amsas, parts. Nine parts for the Nav-amsa for example.
So let's put Mars in the last 3°20- part (=ninth) of pisces. This will put it into pisces in the Navamsa, too, because the last ninth of dual signs give the same navamsha position (vargottama). So it's correct to determine Mars' dignity in the Navamsa as in a friend's sign, because with pisces it's in Jupiter's sign, who is a friend. So this is "real".
Jupiter as lord of Pisces now is in a ninth-part (Navamsa) by himself. But this in no spatial way is related to Mars' ninth-part-position, because the Navamsa is not a three-dimensional thing.
So if you want to determine the spatial connection between Mars and Jupiter (which we need to do to get the temporal friendship) your have to go back to the rasi-chart to do that. Otherwise you would link two mathematical positions in a spatial way, which most likely isn't correct.
Thats the rational behind it.
And it's the same with planetary aspects. Only planets in the 3D-sky (aka "Rasi") can have glimpses to each other. Planets in amsa's can't. They only can rasi-aspect.