The whole process of events isn't actually that complicated...
On Ji Xiaodao's orders, Li Fo contacted Arthur, who was underground in Galos Duke's manor, through those 'special channels' in Sin Duke's Manor, immediately after preparing the delicate treats, and briefly informed him of Guivere Logan's whereabouts.
Arthur, who had always been working with the old man in Merlin's black workshop and was mostly adapting to his new body during this time, almost immediately decided to have the recently acquainted Merlin assist him by giving him an external armor to handle unexpected situations upon receiving this news.
