Questioning one's "mental health" is the new corporate bullying tactic used indiscriminately against employees. Been there and copped that line from a previous employer and their lawyer - didn't stand up at court time but boy did it cause a lot of stress and employment discrimination until the court finally delivered it's verdict in my favour.
And the person behind these false accusations? A so-called professional Human Resources practitioner. I feel for this poor S/O but he really does have to stick to his guns and take it to its natural conclusion.