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.
It is not permissible to describe a Muslim as mad just because he left the masjid straight after the congregational prayer as he would dislike it and it would sadden him. It is also considered backbiting. The Prophet, sallallaahu ʻalayhi wa sallam, defined backbiting as follows: “It is saying something about your brother which he would dislike.” He, sallallaahu ʻalayhi wa sallam, was asked, “What if what I say about my brother is true?” He, sallallaahu ʻalayhi wa sallam, replied, “If what you say of him is true, you have backbitten him, and if what you say of him is not true, you have slandered him.” [Muslim]
In such a case, what is legitimate is to remind that person of the virtue of remaining in the masjid after the prayer in order to recite the legislated Thikr (mention of Allaah) that is said after the prayer. This is what is legitimate in this case, and not describing him as mad.
For more benefit about the merits of remaining in the place of prayer for Thikr after performing the obligatory prayer, please refer to fatwa 275431.
Allaah knows best.