Deleted contacts showing up in skype for business

I have a odd situation.  One of my clients is a very happy user of Skype for business excpet for one thing and I'm at a loss on how to fix it.  They have several old user (termed) user showing up in the contact list.  For example a VP of Marketing has left and instead of show the new VP name it shows the old one.  Is this caused by the computer name or AD or something in EXchange?  I'm not famiular with how skype works, can this be fixed by deleting the chat history?  I'm at a loss.