Upvote:5
Related: Not stamping my foreign passport at US border
The US frequently doesn't stamp passports. Whether a passport is stamped or not may depend on the port of entry, whether you use one of the automated kiosks or not if available to you, immigration status/visa type, the whims of the individual officers or supervisors involved, and other inscrutable factors.
This traveler could look up their I-94 arrival/departure record, which would provide the same (and more) information as a stamp would.