App developer's sending of screenshots to a private actor working as an agent of the gov't did not trigger the private search doctrine, as the second party's search was precisely coterminous with that of the app developers, and no new information could have been obtained in the second search.
[Seems to be an obvious typo in the First Circuit's URL for this on the public-facing page. This link is to the opinion list, which will likely be corrected by now. Search for case # 17-1683]
http://www.ca1.uscourts.gov/opinions
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.