Was Tempelsman a widower when he was seeing Mrs. Onassis?
No.
https://en.wikipedia.org/wiki/Maurice_Tempelsman
Marriage and children
Tempelsman has three grown children by his wife Lilly Bucholz, a woman who had also fled Antwerp with her family. They were married in 1949.[2] His daughter, Rena, is the widow of Robert Speisman, an executive vice president of Lazare Kaplan International Inc. who died on board American Airlines Flight 77, when the aircraft crashed into The Pentagon during the September 11 attacks.[28]
Tempelsman and Bucholz formally separated in 1984. Bucholz later granted Tempelsman a get but the two have never legally divorced due to Bucholz’s devout Orthodox Jewish beliefs.[3]