Question about "final warning letter"

A friend was issued with "final warning letter" months ago, they decided at time to resign at their own accord because of this. It was very unfair to get the letter as it wasnt warranted - too long & complicated to go into. Anyway they are now coming to end of their notice period the company have asked them to stay, my friend wants that "final warning letter" detracted & they have agreed but the employer have said they will still keep it on file. They are planning to issue another letter - yet to be written.

Question is - if you have any "final warning letter" on file - can they still use this against you although they are negatiing it with another letter? IMO it should completely shredded and taken out of file - am I right??

He will be seeking employment law advice but just thought I'd ask here first!

Thanks

Probably better to have the letter AND the retraction together on file.

Even if they don't have it on his HR file, they'll probably have an electronic copy somewhere that could resurface.