Is it obligatory not to err, even by a slip of the tongue, when reciting the Qur’an from memory, and is it permissible to recite it to a friend by having him read a verse and I complete it?

There is no accountability for errors in Quranic recitation during learning; rather, the reward is magnified according to the difficulty. The Prophet, peace and blessings be upon him, said: "The one who is proficient in the Qur'an will be with the noble and righteous scribes (angels), and the one who recites the Qur'an and stutters over it, finding it difficult, will have two rewards." The proficient one is better and has a greater reward. There is no requirement for error-free recitation during memorization playback, nor is there any sin for unintentional mistakes. As for the method of memorization playback, it is broad: one may recite the entire surah or verse by verse, choosing what is most suitable with the help of specialists.