You can add it any time, but before fermentation is usually better. I
never add it before fermenting mead as a heads up, the increase in acid
is too unpredictable. A lot of country wines are like that.

I'm not sure more acid will help bitterness, are you sure he did not
crush some seeds or something like that? If you do add it measure out
the expected amount and put it in in small increments; you can always
add more.

Joe