"Good friend" here meaning "Obvious lover"

Explanation: from Wiki:

Katte was beheaded at the fortress of Küstrin, where the king forced Frederick to watch the execution. However, when he was brought up to be executed, Frederick shouted in French to Katte, “Veuillez pardonner mon cher Katte, au nom de Dieu, pardonne-moi!” ("Please forgive me dear Katte, in God’s name, forgive me.") Katte called back in the same language, “There is nothing to forgive, I die for you with joy in my heart!” Frederick then fell to the floor in a dead faint. Katte’s body was left overnight on the execution scaffold that reached up to Frederick’s window by order of the king.

Upon witnessing his death, Frederick was plunged into deep despair for three days. After that, he never spoke of Katte again nor ever visited his grave. Katte’s body was initially buried in the fortress moat on the king’s instructions. However, at the family’s request, it was exhumed and transferred to the family crypt in Wust. His grandfather was given the executioner’s bill to pay and the executioner’s sword.[7] However, four weeks after his accession to the throne in 1740, Frederick appointed Katte’s father field marshal and raised him to the hereditary rank of count. But since his younger sons died without descendants, the title expired only eight years later.

They must have been roommates