On Thursday, June 27, the Refer-a-Friend quest was given a patch for those still experiencing linking issues. The new player window has been changed to 21 days.
A veteran needs to send their new player another link and that should resolve your the linking issue.
If linking issues continue, please create a Topic in the Help forum.
For more information about the Refer-a-Friend quest, please visit: https://www.bungie.net/Referafriend