Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- We urge all Muslims to get their knowledge from authentic sources.
2- As for Ibn Kathir, he is a great well-known scholar. However, his book contains some non-authentic reports. But no Muslim can degrade him or dare to consider him a liar.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
Perhaps this brother may have in mind the abridged translation of Ibn Kathir distributed by Darus-Salam.
That is not to be confused with the original Tafseer of Ibn Kathir which is in Arabic; it is an authentic work of Tafseer.
However, not everything included in it can be taken as authentic; for as was the custom of the authors such as Ibn Kathir and Ibn Jarir, etc. they cite all kinds of reports in order to cite the various viewpoints. The fact that they cite those reports in no way means that they endorse those views. Often times they express their preferred views clearly.
The abridged translations, however, may contain extraneous materials inserted by the translators. Perhaps your friend may be referring to them. Otherwise, no Muslim would ever dare to call Ibn Kathir a disbeliever.
How can we do so when we know that he was one of the greatest scholars of Islam revered by the Muslims throughout centuries?
I would urge you not to acquire knowledge of Islam from hearsay or those who are not properly qualified to transmit it.
Allah Almighty knows best.