There are mixed answers and resolutions for this inquiry. You can't really tell for sure, but there are a few signs that could indicate if a person is using Viber or not. One is to ask the user directly. It's a common approach used by many. Another is to refresh the Contacts on Viber and see who's listed with the Viber icon. You can't rely on status and availability, because there are many tools nowadays that can fake that.
You should recommend to your friend to reinstall Viber and activate the account again. If this is not working for him, tell him to contact Viber directly because there is also the possibility for his account to have issues.
Related answer: http://answers.informer.com/71007/someone-deactivates-account-disappear-immediately-contact