There is no question she was gathering evidence. The whole issue is not whether she was gathering evidence, her observations are evidence. It is whether you can make a compelling argument for suppressing that evidence and the evidence gathered by the officer subsequent to the peeping tom's evidence gathering.
The state will argue that even if the evidence gathered by the peeping tom was illegal, the subsequent search by the officer based on the now exigent circumstances was legal and that evidence (observations) should be admitted to evidence.
Because Texas has a protection that few states have, and in fact the US constitution does not have.. the right to privacy against unlawful search by citizens you might have a significantly better shot than in other jurisdictions.
The officer was, with full knowledge of the law, knowingly accepting the word of an obviously obsessed and deranged person who was admitting to violating the constitution of the great state of Texas. Joining this person, the officer negotiated his way through private property to engage in a little peeping tom action with his witness... but he was stumped! There were no little kids as far as he could tell. Again, even after viewing the act, he still had nothing indicating a crime.
So now he asks the deranged kid again "are you really really sure" and she says "Uhhuh".. and he still presses the search.
It is my opinion he overstepped his bounds, ESPECIALLY in light of a protection texas citizens enjoy. Not that that affects your life in anyway. You have a lot to think about.