Alhamdulillah. You've asked a very good question. May Allah grant you and your friend guidance and insight into our religion.
The Qur'an is indeed a universal message for all of mankind until the Day of Judgment. That is why Allah has made its message clear and easy to understand.
> And We have certainly made the Qur'an easy for remembrance, so is there any who will remember? [al-Qamar 17]
>(...) And We have sent down to you the Book as clarification for all things and as guidance and mercy and good tidings for the Muslims. [an-Nahl 89]
Still, we find that some parts of the Qur'an can be more difficult to grasp, such as when the Qur'an sometimes addresses Muhammad ﷺ or his companions directly, regarding certain historical events. This however does not mean that the message is limited only to them, rather Allah makes use of even these situations to grant lessons to all of humanity.
Scholars have written different explanations (tafseer) of the Qur'an over time. Of course, this begs the question, why do we need Tafseer if the Qur'an is already supposed to be clear and understandable?
Indeed Tafseer is necessary to fully understand the meanings of the Qur'an. Not everyone can be a scholar - the average reader who doesn't know the context of certain verses or what exactly they refer to, or may not even know the Arabic language, will have a hard time benefitting from it in the same way as somebody who is well versed in these matters.
Iyaas ibn Muawiyah said: "The example of a people who recite the Quran and do not know its explanation is like a group of people who have a written message from their king that comes to them during the night, and they do not have a lamp. Therefore they do not know what is in the message.The example of one who knows tafseer is like a person who comes to them with a lamp and reads to them what is in the message."
The Qur'an itself was not brought as a standalone message, but rather it was given to the Prophet Muhammad ﷺ who recited it to the people around him bit by bit. When they had a question concerning the faith, they would come and ask him directly, and he would explain it to them. Today we find these explanations within the Ahadith which have been preserved. The Ahadith of the Prophet ﷺ (Sunnah) are indeed the best Tafseer of the Qur'an, as the Prophet ﷺ had the greatest amount of knowledge regarding the message of the Qur'an among all men. Most works of tafseer written by scholars are therefore based upon the Ahadith from the Prophet ﷺ and his companions.
Hence the Qur'an stresses the importance of following God and
His Messenger, and therefore Muslims follow both the Qur'an and the Sunnah of the Prophet Muhammad ﷺ.
The Almighty says:
>... until the truth (the Qur'an) came to them along with a messenger making things clear. [az-Zukhruf 29]
In conclusion it can be said that had the Qur'an itself been enough to fully understand the religion, then it could have simply dropped from the sky. But instead it was given to Prophet Muhammad ﷺ so that he may explain and clarify its meanings to the people.