Horne and Corden realised they needed to split before both their careers went completely down the pan. James was always gonna do better than Mat after that point because he's much more talented (it pains me to say!). I don't think Mat had a 'breakdown', where has that ever been said??
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