It was one meal, give it a rough estimate and just keep going. I don't think it's a good idea to not eat the rest of the day, I always binged after too much restriction.
One of the most important things I learned was flexibility. Real life is messy, complicated and unplanned. The old me used to eat something "bad" and then think "well, ****, the day is blown, I'm going to eat chocolate eclairs" The new me just pulls it together and makes the next meal a good choice. Sometimes, I get tempted and eat something I didn't plan to eat. The important thing for me is to not let a bad meal turn into a bad day or a bad week or a bad month. I just recover and keep going!
If you eat 3 meals a day, that's 21 meals a week. One offplan meal is 5%. Eating onplan 95% is DAMN GOOD. I don't think a sliver of everything at a buffet is doing too badly, though. It's not like you went back and filled 3 plates, is it? I used to be able to eat PLATES of food at a buffet

For buffets, I make sure I fill up on salad and then take little dollops of things I want to try. My rule is one plate!
I'm not sure what kind of a crackpot woman would say "everything is fat for free" to you, though. Sheesh, what a piece of work.