Myself, I've come to learn that I don't "believe" or truly take in any opinions of anyone but myself.
My insecurities come from my personal preferences. I realize that my body/physical self is easily appreciated by other humans in a general sense.
Sure, I've been influenced by society and the "norms" it's pushed to the forefront.. but at the end of the day I want to be what my own brain finds attractive. I'm lucky enough to be able to appreciate a wide variety of body types, but I'd be lying if I said I didn't have my preferences.. And that I don't resemble them.
At the same time, knowing that I'll never come close to my fellas preference physically fucks with my head. We had that very conversation tonight. Sure, he finds me attractive 'enough', but like so many people.. I know he looks at other bodies and enjoys them more than mine based on that alone. Superficial as it is, it matters to me.
I guess that's social conditioning as fuck. But the feeling is still real, and relevant.
He can tell me I'm sexy/beautiful/whatever the fuck else all day. None of it sets in until I let it.
There's no lecture, no set of phrases that you can use to make her feel what you want her to feel.
All I can advise: if she's verbalizing her feelings about not being attractive, tell her that she's the least lonely person on earth in that regard. Don't take my advice. But yeah. I'd be brutally honest.
Fuck dudes who gives too many fucks about what someone looks like. She's better off without that shit.