Hi!
Do you mean this user’s contact list is empty, right?
Can you add one user to the contact with no issue?
In this case, you can try to follow these steps to see if it can be fixed. Some customers have the same issue and it worked for them.
*Added the contact back in to Skype for Business
*removed the contact from the contact list
*checked Skype for Business contacts in outlook and the contact was gone
*Added the contact to Skype for Business
*Checked Skype for Business contacts in outlook and the contacts was there.
*Logged out Skype for Business and then logged in again