Jane, June, whatever her name is. She may be making the powerful Roger Sterling happy, but as Don tells him "nobody thinks you're happy, they think you're foolish."
One of the great tensions of Mad Men is old vs. new. Swapping an older wife for a younger one is a midlife crisis not changing with the times. It does, however, highlight Roger's age. His new wife, young enough to be his daughter, makes him act foolish, managing to transform smarmy charm into just smarm. When change divides the '60s into before and after, Roger will be on the wrong side.