My uncle who retired as class I officer in India is now focussing on religion (Islam and compative study with other religion) and have written and published a book on the Tafseel (detaited description) namely HEART AND SOUL OF QURAN of some selected verses of Holy Quran. It is worth mentioning here neither he is regular in offering 5 times prayer nor he is pious in other matters.
My question is whether he is eligible to comment and write description of a Holy Book like Quran and What shall be the minimum qualities / requirement of a person if he/she wants to go ahead in such type of religious work ?

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.

If this man writes well on this subject, then it is permissible for him to write on it, as this is a good act in itself. The fact that he commits some sins and that he is neither pious nor practising, does not prevent him from serving his religion in this field; of course, he is rewarded for everything that he does. Allaah Says (what means): {So whosoever does good equal to the weight of an atom (or a small ant), shall see it. And whosoever does evil equal to the weight of an atom (or a small ant), shall see it.}[Quran 99:7-8]

Moreover, it is confirmed that the Prophet said: ''Indeed, Allaah may support this religion (i.e. Islam)  even with a disobedient man.” [Al-Bukhari and Muslim]

There is no doubt that that man must observe the religious obligations and stay away from prohibitions. He should be an example and adopt the morals of the Quran which he is writing about. He should be advised in this regard in a soft and gentle manner. A person who may influence him should advise him especially in regard to being negligent about the prayer as this is one of the most serious matters as discussed in Fatwa 85264.

If a person wants to write on any Islamic science, especially the Quran, he must be knowledgeable about this science and about other sciences that may help him understand this science.

Finally, we advise you to show this book to the scholars in your country and if it contains any mistakes, you should bring that to the attention of the writer so that he would correct them. If he does not do so, you should do what the scholars advise you to do in this regard, as it is advice. Tameem Ad-Daari narrated that the Prophet said: "Religion is advice". They said, "For whom?'' He replied, "For Allaah, His Book, His Messenger and for the leaders of the Muslims and their masses." [Muslim]

Allaah Knows best.