What does sir launcelot do to aid king bagdemagus in du lake?

Sir Launcelot does not aid King Bagdemagus in the story of the "Du Lake," which is not a known story or tale in Arthurian legend.

It is possible you are thinking of a different story or misremembering some details.

Could you please provide more context or details about the story you are referring to? This would help me understand what you are asking and provide a better answer.