When a brother is wearing something like t-shirt or top and hasn't tucked it into his jeans and tightened his belt or something similar part of his 'awrah such as the top of his bottom and bottom of his back get exposed in rukoo and/or sujood. Should I tell him this after the prayer (in a polite way) so that he is more careful in future and that he will need to repeat the prayer. However if I don't tell him he wouldn't know which would mean his prayer is valid or not?
On the authority of Abu Sa’eed al-Khudree (may Allah be pleased with him) who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say, “Whosoever of you sees an evil, let him change it with his hand; and if he is not able to do so, then [let him change it] with his tongue; and if he is not able to do so, then with his heart — and that is the weakest of faith.” [Muslim]
According to this Hadith we all should stop wrong things by different ways. You should tell him if you care about him(in polite way definitely) But if u think that the person will mind that then try to tell about him with his near friend or someone.May Allah give us right path to follow.Amen