That really depends on closeness and your personal experiences. If you're looking for things to help yourself I can talk about that. I have a history of weight based dysmorphia and at times have been unable to look at my stomach while at exactly the middle of healthy weight because my organs felt like fatness. I also have complicated relationships with other aspects of my body as well, but not at risk of disorder or anything, but insecurity nonetheless plays a role.
A casual acquaintance in normal circumstances: "that sucks buddy, honestly I think you look fine/great/whatever but not a lie" You can't help beyond being the choir of "we aren't thinking negatively about your body".
Anybody else, you can relate it to your own experiences with it of yourself or a loved one. A loved one of mine has a history of anorexia, so sometimes I'll relate to people on that rather than talking about my experiences if I don't feel comfortable with discussing it with them.
Ultimately, you need to understand where the person's insecurities and discomfort come from and to be someone they're open to help from to really help. Partners will often struggle to help beyond just showing avid attraction, but at the same time, I've had my fair share of sex with my shirt on or refusing sex because it would involve exposing my stomach.
I will say that having a friend give unusual compliments did help my self esteem in other ways. And having trusted friends give compliments can be valuable. But at the same time, unsolicited compliments about common locations of insecurity are generally not warranted and if you don't know they are they're better not said. However if someone you know is overshooting their goals to the point of concern like becoming underweight you can say something