Google talk/hangouts question

So I have a question for a friend (he doesn't post here). His wife is acting very suspiciously and he has found a few numbers but as of yet is still gathering evidence - as we all know, if he confronts too early then she will be that much more careful. He knows she uses google hangout - as this keeps things off of texts - but she hides the phone when he comes in the room. Now, he knows her email and signin, if he added the google hangout app to his phone under her login info, would those chats show up? I think that if signed in under one account, the info can go to every device that has it - but here is his question: will it show if he doesn't add her phone number? He won't do that because it will send a code to that number to confirm it. Is he out of luck or will this work using just her email address?

Evidence of an affair can really put her in a bind - think workwise...

 

I use hangouts as that is the default messenger for my Nexus phone. To my knowledge, the only way they will be able to view it is if that can get the phone. Downloading the app and signing in will send a notification to the person's gmail account, letting them know it was logged in.