I suspect that in the current quiet time for travel, they are doing audits, especially of multiple claims via "Delay Genie" and similar apps . If the claim or claims date from a year ago, if fraud is suspected, the only way this can be dealt with is via the Crown Court at trial on indictment. This is because the six-month period has passed for prosecutions in the Magistrates' Court.
The person in question should carefully consider their position. If they have indeed even possibly submitted a potentially dubious claim, (regardless of value), they ought to obtain legal advice. If they have used a smartcard, then they REALLY need to consider their position, because an audit will likely compare tap in and tap out times at ticket barriers / on board inspection taps / platform validator machines against the claimed "delayed" journeys.
If the person has had a letter from the TOC (wouldn't be Abelio Greater Anglia by any chance?) it may have offered the opportunity to repay any claims the claimant now feels they made in error. If the sum in question is small, they may (especially if the claim or claims were made in error) decide to minimise the cost by paying up.
If they have had such a letter possibly they should consider whether, on mature reflection, the simplicity of putting through a claim with a few clicks meant that they decided to put through a claim for the train before/after the one they took, or on a day they worked from home, etc. I am, of course, NOT suggesting this is the case, but some people justify this to themselves because “the railway has loads of money”, “tickets are too expensive”, or “I always get delayed 10-14 minutes so just miss out”. But doing so is fraud.