There really is not gentle way of saying "Dude you're a slob." This is on the same level as flush the toilet, wash your hands after you crap etc. It will however sound way better coming from a friend than his partner. There is stuff close friends can say to each other that intimate partners probably shouldn't. When I was steadily climbing to 300+ pounds hearing it from the wife would have had no effect. When a good buddy tells you "Dude you're ****ing fat" It feels very different then when your partnter says it.