I would look into a day pass for a lounge (or see if your friend has already has lounge eligibility like something through priority pass but I would assume your friend would know about that already). Lounge Buddy is good for looking up lounge info.
Could also eat at a restaurant, shop the Disney/Universal/other stores in the airport, bring or buy a book and read, bring an iPad or laptop to watch, etc.
6ish hours might be enough time to leave and come back (especially if they have TSA pre-check but I'm assuming they don't based on what you wrote) but I'm not sure what they would even want to leave to do.