The link in the video appears to download a valid version of Whatsapp. However, as a general rule, turning on "Unknown sources" in the device settings opens up a gaping security hole in the device - simply clicking on unsuspecting links can install malware sourced from outside the Google Play store.
For people who are not security savvy, this is a dangerous suggestion.
At the very least, any member who follows the advice on the video and installs the non-store version of WhatsApp should also be told to turn off "unknown sources" setting as soon as Whatsapp is installed.