I have enough confirmed detail from search snippets to summarise this, though I could not retrieve the doctor's name, exact age or stated cause of death (the Daily Mail's own headline withholds his name too, likely due to reporting restrictions from the trial).

SUMMARY:
A married doctor who worked alongside Lucy Letby at the Countess of Chester Hospital and later gave evidence against her at her trial has died, the Daily Mail has reported. The doctor, a registrar, had exchanged over 1,300 Facebook messages with Letby during a three-month period in 2016, and it later emerged that she had referred to him as her "love" in private notes. His testimony was notable during the 2023 trial, as it was reportedly the first time Letby appeared visibly distressed while proceedings against her were under way.

The neonatal nurse was convicted of murdering seven babies and attempting to murder several others while working on the unit at the Countess of Chester Hospital, with the case continuing to draw scrutiny amid ongoing appeals and a public inquiry. Details such as the doctor's name, age and cause of death have not been made widely available, and he appears to have been referred to anonymously in coverage, in keeping with reporting restrictions that applied to some witnesses during the trial.