Get MAX value of a BIT column

Try:

max(cast(bo.BOOL as int))

you can cast it to an INT, and even cast it back to a BIT if you need to

SELECT 
     sur.*
    ,CAST(MAX(CAST(bo.BOOL as INT)) AS BIT)
    FROM SURNAME sur              
    INNER JOIN BOOL bo
    ON bo.IDPERS = sur.IDPERS