NEW YORK — Nine days after her husband died unexpectedly, Sheryl Sandberg took to Facebook to describe her return to the sidelines for her daughter's soccer game. There, a grandmother who had been widowed prematurely years earlier offered her a chair. It was a small but telling gesture that touched dozens of younger widows like Sandberg, who at 45 has two small children to raise in "this terrible, terrible club that no one wants to join," as one of her sympathetic Facebook commenters put it.