The question of "damage done" is only relevant to appropriate punishment - not whether or not the act itself violated the Code. If no damage is done, the Code may still have been clearly broken, and vice versa.
Oh! "legal affairs" vs "illegal affairs", not vs "personal affairs" - got it. Doy.
Oh, so the interpretation is
"The Tribunal has ruled that it's illegal to scry upon a non-magus, if by doing so you learn about the magus's activities, unless it's on a bandit grog who was a witness to those activites, and/or they later proved to be questionable." (Could you tell me where to find the original of this?)
"Illegal" is illegal is illegal - the circumstances may minimize punishment, but they do not change the fact of the matter.
And without having read the passage, I'm going to go out on a limb and guess that to "forfeit immunity" a mage must actually have broken the Code, not be suspected of breaking the Code - proving at a later time that he ~may~ have bent the Code (in the eyes of one maga) is not justification for that maga scrying before she knew whether or not he had broken the Code.
By your own admission, she did not know the situation before she scried - so how could she know he had forfeited immunity??? You cannot rationalize an illegal search by the evidence that illegal search reveals. It's still illegal.
What, if it turned out he had not, suddenly those same actions become more illegal? Anyone can scry on anyone, so long as that target can then be proven to have broken some Code?
Further, I hate to point this out, but magi are not the arbiters of "legal vs illegal" in the Order. Tribunals are. So your argument that he broke the Code would have to be supported by Tribunal Judgement first - and until then, she is as justly accused as he (and maybe more so).
(Wouldn't surprise me if forfeiting immunity is a bit more specific and difficult than you're painting it, requiring a previous accusation to or ruling by a Tribunal, or even authorization of House Guernicus, and not just vague suspicions and unjustified actions by any nosy mage who wanders by.)