Two of my colleagues and I found a book entitled Shams Al-Ma‘aarif Al-Kubra by Al-Booni and the Mus'haf (printed Quran) in another colleague’s room. Some of its words had been distorted with a pen with some foul words. We took the two books and confronted him with them. He confessed that the books belonged to him and he swore that he would repent to Allaah The Almighty and asked us to conceal his sin.
We burned the two books in front of him and concealed his sin, hoping that his repentance was sincere. However, ten days later, we found him again writing the same words on the Mus'haf and thereupon we decided to inform the concerned authorities and tell them about the past incident. As a consequence, he was arrested and he confessed to everything and we were summoned to testify.
What is the Sharee‘ah ruling on this person? Did we do the correct thing under the Sharee‘ah?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Writing foul words in the Mus'haf is an evil crime; it even indicates disbelief and apostasy from Islam. It is one of the deeds of wicked sorcerers through which they summon devils. Satan is keen on enticing man, leading him to deviation and taking him away from his religion. Shams Al-Ma‘aarif Al-Kubra is a book of sorcery and falsehood. It is impermissible to read it or to act on what is written in it.

You were right when you burned the two books and concealed his sin, hoping he would sincerely repent. It was also correct to inform the concerned authorities after he had returned to his deviation and falsehood.

If it is established that a person uses sorcery or desecrates the Mus'haf, he is asked to repent and if he does not then he must be killed. Imaam Maalik and Imaam Ahmad were of the opinion that a magician is to be killed without first being asked to repent. This is because the Companions killed magicians without asking them to repent, and because the knowledge of sorcery is not removed by repentance.

It was narrated on the authority of Hafsah that she killed one of her bondmaids because the latter had bewitched her. [Abd Ar-Razzaq] It was also narrated on the authority of Jundub that he did the same. Imaam Ahmad mentioned that killing sorcerers was narrated on the authority of ‘Umar, Hafsah and Jundub .

Similarly, Bajaalah narrated that ‘Umar wrote and told him to kill every male and female sorcerer. [Al-Bukhari and ‘Abd Ar-Razzaaq] 

Allaah Knows best.