McMillan posted the photo of the pair Saturday with the caption, "Best #friends #no Makeup #girl-time."
McMillan was the mastermind behind Aniston's famed "The Rachel" haircut back in her early days on "Friends."
"I know that was Jen's least favorite hairstyle because I hear about it every time I style her hair," the stylist told Marie Claire in June.
Aniston has already asked McMillan to style her hair for her wedding to fiancé Justin Theroux. She said she doesn't want to boast an undo for the affair.
"I like loose and natural hair because it goes against the formality of the dress," she told People. "Loose, like 'I just got out of bed, maybe had a romp' hair."