Kudrow, 55, surprised Cox, 54, during her interview with host Ellen DeGeneres on Tuesday's episode.
Cox hosts the new Facebook series Nine Months with Courteney Cox and plans to join Instagram to promote the show. DeGeneres recreated the Central Perk set from Friends for Cox to take her first photo.
"I wish Lisa Kudrow was here because that would be fun," DeGeneres said before Kudrow appeared.
Cox and Kudrow played Monica Gellar and Phoebe Buffay on Friends, which had a 10-season run on NBC from 1994 to 2004. Jennifer Aniston, who portrayed Rachel Green, said in the September issue of InStyle that she's discussed a revival with the actresses.
"Before that show ended, people were asking if we were coming back," the star told the magazine. "Courteney and Lisa and I talk about it. I fantasize about it. It really was the greatest job I ever had."
"I don't know what it would look like today, but you never know. So many shows are being successfully rebooted," she added.
Kudrow previously discussed the possibility of a revival during an episode of Conan in January 2018.
"Something should be done, I don't know what," she said. "They're rebooting everything, but I don't know how that works with Friends."