Sally is a woman who Ross temporarily meets in "The One With The Pediatrician" in the waiting room outside Dr. Gettleman's office.
They have a brief conversation about single parents when she sees no ring on Ross' finger. When it's Ross' turn to see the doctor, Ross tries to scheme himself out of embarrassment, sees a little boy and says to him "Come on, Ross Jr. It's time to go in." and he gets up and goes to Sally and it turns out the boy is her son. Then Sally's daughter asks her mother what's wrong with Ross and Ross shouts "Hey, I helped you find Waldo!" and she hides her face in her mother's chest, much to Sally's confusion and bewilderment.