If I see a worshipper's bare back in prayer should I tell him?

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?