First of all, we would like to stress the fact that forgetting the Qur’an after memorizing it and showing negligence in re-memorizing is a sin. If one strives hard to review the Qur’an regularly, there will be no sin on him even if he forgets some of it. The blame is on those who neglect it and fail to review and read it regularly.
The prominent Saudi Islamic lecturer and author, Sheikh Muhammad Saleh Al-Munajjid, states the following: “Undoubtedly forgetfulness is something that is natural in man, and man is only called Insan because of his forgetfulness.
Usually, this varies from one person to another, according to the differences in the strength of the memory that Allah has created in His slaves.
It is also known that the Qur’an “escapes” from the heart if the Muslim does not constantly and regularly review what he has memorized.
The reason for this may be that it is a test for people’s hearts to show the difference between the one whose heart is attached to the Qur’an and thus regularly recites it, and the one whose heart is attached to it only whilst memorizing it, then he loses interest and forgets it.
The reason may also be to give the Muslim a strong motive to recite the Qur’an more frequently so as to attain the immense reward for every letter that he recites. If it were the case that he could learn it and never forget it, he would not need to read it frequently and then he would miss the reward for reciting and reviewing it regularly. Fear of forgetting it will make a person keen to recite it so as to gain more reward from Allah. For every letter you recite you will have one good deed or Hasanah, and the reward for one good deed is ten like it.
The Messenger of Allah, peace and blessings be upon him, urged us to read the Qur’an regularly lest we forget it. He warned us against being negligent in this regard, as was stated in numerous Hadiths, including the following:
Ibn `Umar, may Allah be pleased with him, quoted the Prophet, peace and blessings be upon him, as saying: “The likeness of the one who memorizes the Qur’an is that of the owner of a hobbled camel. If he tends to it regularly, he will keep it, but if he lets it go, he will lose it.” (Reported by Al-Bukhari)
It is well known that if a camel escapes and runs away, it takes much efforts to recapture it. Similarly, if the person who memorizes Qur’an does not regularly review what he has memorized, he will lose it and will require a great deal of effort to get it back.
So, whoever strives to memorize the Qur’an and recites it regularly, it will be made easy for him, and whoever turns away from it, will lose it.
This is what encourages us to constantly review what we have memorized and to keep on reciting it, lest we forget it.
The Hadith reported by Anas Ibn Malik, may Allah be pleased with him, quotes the Messenger of Allah, peace and blessings be upon him, as saying: “The sins of my Ummah were shown to me and there is no sin greater than that of a man who was given or was helped to memorize a Surah or an Ayah of the Qur’an and then he forgot it.” This is a weak or Da’eef Hadith, according to Al-Bukhari and At-Tirmidhi.
In his book Al-Itqan, Imam Al-Suyuti states:
Forgetting the Qur’an is a major sin, as was stated by An-Nawawi in Al-Rawdah and others, because of the Hadith “I was shown the sins of my Ummah…”
One of the best ways to help oneself remember what one has memorized and to make it firmly entrenched is to recite it in one’s prayer, especially during night prayer or Qiyam Al-Layl.
Thus, if you strive hard to review the Qur’an regularly, there will be no sin on you even if you do forget some of it. The blame is on those who neglect it and fail to review it and read it regularly.”
In light of the above Fatwa, we can conclude that you should strive hard to review what you have forgotten, keeping in mind that the Hadith from which your friend drawn his evidence is a weak Hadith. However, this does not rule out that forgetting the Qur’an is a sin that incurs punishment in the Hereafter.