They might be referring to this
Horne appears in the film Lesbian Vampire Killers and made his stage debut in a 2009 revival of Entertaining Mr Sloane at the Trafalgar Studios. On 3 April 2009 it was reported that Horne collapsed on-stage during a West End performance of the play. He remained on-stage until medical assistance arrived, at which point he was taken to hospital. A statement released soon after said that he was exhausted after working 'flat out' for 3 months and it was later revealed that he was suffering from a virus
But physical exhaustion and mental breakdown are two very different things.