Is it obligatory to inform colleagues about a mattress becoming impure due to involuntary urination on it, given the fear that their prayers might not be accepted, and how can they be informed of the situation without embarrassment?

Concealing the impurity of a mattress upon which one prays and steps is not considered disbelief (kufr); rather, it is a sin from which one must repent, because it falls under the category of enjoining good and forbidding evil. It is obligatory to inform whoever intends to use this mattress of its impurity, or to purify it if possible without hardship. One may use allusions (tawriya) and circumlocutions (ma'areedh) to avoid lying. The prayer of someone who prayed on the mattress while ignorant of its impurity or having forgotten it is valid and does not need to be repeated. The one who informs them is not obliged to tell them about their past prayers.