The singer is said to have stayed overnight at their £5 million marital home in Godalming, Surrey.
The pair divorced in September after four years of marriage, following revelations that the Chelsea footballer had cheated on the 'Fight For This Love' singer several times.
A source close to the couple said: "They were saying what a special night it was. They were mucking about like old times just playing pool, smoking and having a few drinks.
"There wasn't any awkwardness between the two of them and they seemed very comfortable together."
An insider attributed Cole's return to her ex-husband to her axe from The X Factor USA and her split from dancer Derek Hough.
"Cheryl's life has been turned upside down," the source added. "A few months ago she was happy with Derek and thought she would be a superstar in America. But everything's changed.
"She feels very unsettled and Ashley can provide her with comfort and stability. She's confused about her future and wants some time to hide away and think.
"She knew Ashley was going away so it made sense to be there. They spent the night together and got on brilliantly. Who knows what will happen when Ashley gets back from his holiday."
Speaking about their marital home, the friend said: "It was Cheryl's dream home. She imagined she would spend the rest of her life there happily married with Ashley and planned to bring up a family there too."
> Cheryl Cole 'to take break from showbusiness'