Background: Family and friends of nursing home residents, broadly defined to include relatives by blood, by law, and close informal relationships, play an important role in enhancing person-centred care by providing direct support and advocating for residents. While their experiences have received attention, there remains a lack of validated instruments specifically designed to assess person-centred practice from their perspective. This study aimed to develop such an instrument for long-term care. Methods: Drawing on a unified theoretical framework, the Person-Centred Practice Inventory – Family/Friends (PCPI-F) was developed by adapting validated instruments designed for staff and residents. The PCPI-F was designed to capture two distinct perspectives: an “Individual” perspective, reflecting the family member’s or friend’s own experience (18 items), and an “Advocate” perspective, reflecting their role in representing the resident’s interests (15 items). After pretesting and refinement, data were collected via a cross-sectional survey of 461 respondents from 34 long-term care facilities. Both perspectives were tested in separate measurement models using confirmatory factor analysis and reliability analysis in RStudio. Results: Both models demonstrated satisfactory fit and factor loadings after minor, theoretically justified modifications. The instrument demonstrated strong internal consistency and construct validity. One item with a low factor loading was removed from the “Individual” scale, resulting in a 17-item version. In the “Advocate” scale, six items had “cannot judge” response rates of 10% or higher, reflecting varying degrees of care involvement. However, overall model fit and parameter estimates supported retention of all items. Conclusions: The PCPI-F is a structurally validated instrument for assessing the views of family members and friends on person-centred care in long-term care. It complements existing instruments and fills an important gap in multi-perspective evaluation. Further research is recommended to clarify roles and optimise item relevance.