Either the manager or his assistants failed in _____ duty.

When a Plural Noun and singular Noun are joined by or or nor, the Pronoun must be in the plural .
- Either the manager or his assistants failed in their duty.

When two or more Singular Nouns are joined by or .either...... or, neither...... nor , the pronoun is generally Singular; as
- Rama or Hari must lend his hand .
- Either Sita or Amina forgot to take her parasol .


[Source: High School English Grammar and Composition BY Wren and Martin.]