Can a non-Catholic wear a scapular?
A scapular is not fashion, it represents a particular devotion of the person wearing it. Some scapulars are part of belonging to an actual confraternity. Some forms of devotion attached to scapulars also come with indulgences.
If someone is seeking indulgences and engaging in Catholic devotions then the real question is why is he or she a non-Catholic instead of a Catholic?