I believe this is actually a system error or the person on the other side is using some kind of privacy tweaker which allows her to change the online status presence. This is a known practice among users of these social networks. In fact, I am using one as well to hide my online presence from other users as well as Sent/Delivery status of the messages received through the WhatsApp application.
It's best to call on her mobile number to ask about this situation. If you get in touch with Viber support, I believe they will say it's actually a simple server error.
