"All revenue was earned through 1099's."

Except, unless this is a medical service provider, the S Corp isn't even going to receive 1099 from customers. There is no requirement for that informational reporting.

However, your client should already be tracking and reporting all business income, even if no customer ever sends a 1099. Unless this is an investment earnings of some type?

Because there is no reason to expect nor rely on 1099-Misc or -NEC for an S Corp, when it's not required, and it's not their full financial revenue source of information.

So, before you amend for something they forgot to give you, have you confirmed the value isn't already included in their business records you used for the initial preparation? Or, have you confirmed this person only has you file against the information forms filed by others, and has in fact been ignoring their factual revenue?