Finding violations of the symmetry constraint
To find the rows, use a left outer join:

select 
    a.Friend1ID, a.Friend2ID, b.Friend1ID, b.Friend2ID 
from
    Friends a left join Friends b 
        on (a.Friend1ID=b.Friend2ID and a.Friend2ID=b.Friend1ID)
where 
    b.friend1ID IS NULL ;