"gave me a new 1099-NEC"

I wondered the same thing others did. Is this client not revealing their activity in total, not revealing it at all and thinking it only matters when there are 1099-NEC informational forms issued?

"Can I file a 1040-X today?"

Shouldn't you ask them for all the numbers, now that you learned they are missing info? It's not just this one form's amount, I bet. If they don't have complete records, make sure they know that ignoring info which is discoverable (a word I use in class) means they need to figure out the missing info and provide it so that this is the only amendment you need to do for them.

I don't like working with people who hide info.