Is there anything wrong with saying "May Allah accept your supplication" to someone who does not pray or fast, after they said to you, "May Allah admit you to Paradise"?

Abandoning prayer, while acknowledging its obligation, is one of the gravest sins. Some scholars even consider the one who abandons it to be a major disbeliever, an apostate from the faith. The sin escalates if it is accompanied by abandoning fasting.

Regarding your statement to your colleague, "If Allah accepts your supplication": - If your intention was the responsive "then" (إذن), as an affirmation of his supplication, then there is nothing wrong with it. - If your intention was the conditional "if" (إذا), implying that it is unlikely his supplication will be accepted due to his disobedience, then there is nothing wrong with it in terms of not lying. However, it is not appropriate because it is a judgment against Allah Almighty and a statement about Him without knowledge. For Allah may forgive all sins and answer the supplications of disobedient people. It is narrated in a Hadith that a man said: "By Allah, Allah will not forgive so-and-so," whereupon Allah Almighty said: "Who is it that swears by Me that I will not forgive so-and-so? I have forgiven so-and-so and invalidated your deeds."

Therefore, a Muslim must avoid ambiguous expressions; let him speak good or remain silent.