It appears that the ascorbic acid is superadditive with whatever it is in the coffee that is also a developing agent.
Nb I say appears, and nb nobody really knows what the developing agent(s) is(are) in coffee ... Though caffeic acid is a current favourite candidate.
It is possible to develop flim in coffee (+ alkali) alone, but the developing times seem very long and the results very low contrast
a long time contributer to apug mentioned in a thread several years ago
that he uses coffee with nothing added, no alkali, nothing but coffee ...
and he is always pleased with the results from his development.
if i can, i'll try to dig up the thread and read it again, but it might be so burried
that it is impossible to find ...
found it --
(there was a url link here which no longer exists)