Too many Muslims have become far too comfortable throwing out these disgusting accusations.
The Prophet said: “If a man says to his Muslim brother: “O, Kafir! (Disbeliever)”, then one of the two will have Kufr drawn on him.” [Al-Bukhaari]
In another narration, the Prophet said: “If a person says to his brother “O, Kaafir! (Disbeliever)”, then it is like killing him.”
It doesn’t matter if they have sinned in the past and have repented. You do not have the right Islamically to do this.
Allah SWT is the Most Merciful, the Most Kind, the Most Forgiving.
That’s all. May Allah SWT guide us all.