Not exactly sure what the app adds to the "Find My iPhone" I have been using (if that wasn't clear, sorry). I have been able to find their phones on a computer using iCloud and a Windows computer, since they have "Find my iPhone" turned on in iCloud on their phones. I've not tried to find their phones on my iPhone, which has a different iCloud and Apple account number. So I guess to be specific, if I download the app to my iPhone and log in using their iCloud account number, I would be able to see where their phone is and it wouldn't mess up my phone because I logged in using another account number? Sorry if I'm confused about this.....