If an Israeli citizen's passport was not stamped upon leaving the U.S. will she have a problem return to the United States?
Me as a German, I entered and left the US dozens of times over the years and never got an exit stamp. Authorities know by the airline database that you left... so, no stamps on leaving... at least in the US. Most other countries do though.
It has been said already that when leaving by air, the US does not stamp passports. When you check in, the airline takes the stub of the form you filled out when entering the US, and which usually gets stapled to the page of the passport where the entry stamp was placed.
Sometimes, things get messed up, and your exit is not properly recorded. In this case, it can be very helpful to keep the boarding pass of the flight leaving the US, and have it ready if needed.