Попробуйте:
def fix_roles(h)
user0 = h['person']['user'][0]
user0['roles'] = user0['roles'].join(', ')
end
[Изменить] Например:
h = {"person"=>{"user"=>[{"username"=>"foo", "status"=>"Y", "roles"=>["accounting", "sales"]}]}}
fix_roles(h)
h # => {"person"=>{"user"=>[{"username"=>"foo", "status"=>"Y", "roles"=>"accounting, sales"}]}}