After their last film together, It's Always Fair Weather, in 1955, they never worked together again. Stanley had already been on his own directing other movies and wanted to get out from behind Gene's shadow. Gene got all the credit and screen time when they were together and Stanley simply grew up and became resentful of Gene. I don't blame Stanley at all. It was his time to shine. He, eventually, outshone Gene in the directors seat and had a brilliant career behind the camera working with Cary Grant, Fred Astaire and Audrey Hepburn in many movies.
He and Gene never worked together and rarely spoke or saw each other again until 1991, five years before Gene's death.
Donen, on Kelly in a 1992 interview talking about the split after It's Always Fair Weather: "I thought about quitting, and I almost did. Co-directing is a nightmare. Co-anything is a nightmare.... It's especially unpleasant and very difficult. To work with somebody particularly somebody who is so concerned with his image, was impossible." "I'm grateful to him, but I paid him back the debt, ten times over. And he got his money's worth out of me." Not only did Donen never work with Kelly again, but he never worked with Arthur Freed, MGM or Comden and Green.
So sad that this great team split on bad terms.