Hi!
yes, the sentence is ambiguous, but actually it's quite natural... I could come up with some ways to make the meaning of the sentence undistinct, but somehow they do not sounds as "jatorra" as the original one.
The one you propose needs a little correction: you need the particle -KO to
lagun baten zintzilikatutaKO argazkia
or perhaps
lagun baten argazkia ikusi du, balkoitik zintzilik.
That sounds perfectly jatorra, too, but one could argue that this time the ambiguity has moved to the Nork case... was the observer hanging from the terrace, when he saw his friend's foto?
if we really want to push it perhaps we coul go with
balkoitik zintzilik zegoen argazkia ikusi du, lagun batena.
but, I'd say it introduces a different flavour to the sentence, as if the narrator wants -for style reasons perhaps- to actively conceal that the foto was the one of a friend untill the last moment it was grammatically reasonable.