It appears (verified) in Mitch Albom's Tuesdays with Morrie (1997) but not attributed to Benchley.
Matthew Hahn, president of the Washington "Lost Locomotive" Chapter of the RBS, reports that the line appeared in Irish playwright Hugh Leonard's 1978 play Da.
It appears (verified) more expansively in Robert Woodruff Anderson's 1966 play (and 1970 movie) I Never Sang for My Father:
"Death ends a life, but it does not end a relationship, which struggles on in the survivor's mind torwards some resolution which it never finds."This occurrence also is without attribution to Benchley.
Does anyone care to research this any further, or shall we say it is a Robert Anderson quotation with no Benchley association?