I did a little googling and looked at one HTH algaecide's SDS. They make many types. Anyway.... it looks like the ammonia compound is about 10%. So one gallon of 10% ammonia needs 10 gallons of 10% bleach to neutralize it.
I would caution him not to dump it all in at once. It could bleach the liner. Give it time to mix and react. Pour a gallon into the return stream, brush things a little, wait fifteen minutes, repeat. And he might already have neutralized some,, so maybe stop at 7 or 8 and give it a day's rest and then see how it looks and what the test results look like, bleach and ammonia will create a lot of CC.