Intercepting communications at any level is illegal. No ifs and buts.
It's "legal" when the government says it is
And not possible, at least for the OP from what's been gauged during this short interaction.
http://support.viber.com/customer/portal/articles/1600146-are-my-messages-secure-#.U8-4YoCSzMM
He could install a spyware app on the phone that sends screenshots every x seconds?