Does the brother's statement represent a call to secularism, and is one who is influenced by it sinful? And what is the proper action to take regarding it?

One must understand the comprehensive relationship between religion and life in all its aspects. For in religion there is no such thing as important and unimportant, but rather important and more important. It is based on giving sincere advice to Allah, to His Book, to His Messenger, and to every Muslim, while emphasizing the importance of diversifying the da'wah discourse and considering the state of the addressed individual. Also, that speaking about religious matters is not exclusive to anyone, and that whoever calls to guidance shares in the reward with those who follow him, and whoever calls to misguidance shares in the sin with them. The conflict between truth and falsehood is part of the mutual defense mentioned in the Quran.