Filing an Amendment - Help

Brother did his taxes on turbo tax and screwed up - ended up including a document that he should have given my parents for them to include on their return - and he ended up owing money. I filed an amendment and it went from him owing roughly $1200 to him being owed roughly $1800. This amendment was mailed and will obviously take time for it to register.

This morning he sees that the IRS has credited his account for the $1200. So now a few questions:

1) Never filed an amendment before, so not sure how this works. But will they ignore it or will they honor it?

2) What kind of time frame is he looking at now for his amendment to come through?

3) Should I make an appointment with the IRS in New Orleans?

Like I said, never been in this situation before so am looking for someone who has. TIA