If she does everything at the airport, she will be fine, a fair percentage of people do this, it does add a bit of time, so if she can use VerifFLY (via the PNR entry system) or Salesforce (via the link upthread) then that has several benefits
- it reduces the time needed at the airport (she can then just scan her passport at the machines and get a boarding pass there, so no queues - this bit is nothing to do with the pandemic)
- it checks that she hasn't made a fundamental mistake in the requirements, notably using the NHS cards rather than the NHS barcode for vaccination proof.
- it may just allow pre-printed boarding pass / app boarding pass.
But if she didn't do this, then it's fine, allow a few extra minutes at the airport. Tens of thousands of people are going to USA every and the requirements now are fairly light touch. When flying to USA during the pandemic and just on the Special Interest scheme then BA agents checked every detail carefully, now it's much easier.

Specifically for the attestation, for BA specifcally there is a barcode that agents can present which allows people to attest online at check-in.