Write SQL query to solve given problem: What is the average score of the establishments owned by the owner with the highest number of establishments?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT AVG(T1.score) FROM inspections AS T1 INNER JOIN businesses AS T2 ON T1.business_id = T2.business_id GROUP BY T2.owner_name ORDER BY COUNT(T2.business_id) DESC LIMIT 1