Ingrid Loyau-Kennett, 49, was praised by Prime Minister David Cameron for her actions shortly after Fusilier Rigby, 25, was killed near his barracks in Woolwich, London, in May last year.
But she is now under investigation following claims a Tesco worker was racially abused at a store in Helston, Cornwall, on Friday, according to a Sunday newspaper.
A spokesman for Devon and Cornwall Police confirmed the force was investigating an allegation of "verbal racist abuse" at the Tesco store.
A Tesco spokesman said: "We are aware of an incident which took place at our Helston store on Friday. Colleagues at the store are helping the police with their inquiries."
Ms Loyau-Kennett received national acclaim for calmly talking to armed Michael Adebolajo and Michael Adebowale after they murdered Fusilier Rigby in the street.