This depends entirely on the person. I wouldn't just go up to some random person in the street and tell them they stink; that's totally unnecessary and not really my place, IMO. xD But, if someone who's actually part of my life, then yeah, I'd mention it. Just depends on the person. I wouldn't say it to someone who would really be hurt by it. That said, if someone really smells that badly, it could indicate that they're not looking after themselves too well, as many of us here will know! (depression)