If all other applications work normally, then the problem concerns the QQ Application. The first thing I would recommend is a fresh installation on both devices (yours and your friend's). Secondly, initiate a new video session and if the error persists, then the application is bugged. The next step is to report on this problem to the developer. He might ask you to send bug reports and other related information in order to properly fix the problem. Alternatively, install the latest version from the App Store and then chat again.
Download application: https://itunes.apple.com/us/app/qq-international/id710380093?mt=8
Contact info: email@example.com