Worried about best friend’s girlfriend

I really hope this is alright to post, I just need some help since I know nothing about eating disorders and I feel like I might be overreacting, since no one else says anything. TLDR: My friend’s girlfriend has been losing weight really quickly and no one seems to say anything, but I think her habits are a bit worrying. Basically a bit ago my best friend of many years introduced me to his girlfriend, lovely girl. She hangs around with us a lot which is cool, she’s fine enough, just very quiet. Now at first everything seemed just fine. But I’ve noticed a really sudden and really rapid decline in her. Almost out of nowhere, her snapchat was filled with her talking about dieting and such. Which I’m not going to judge anyone for wanting to diet, go for it if it makes you happy. She was sharing meals and stuff she was making, which looked small but it was fine. But she’s only been getting worse. She posts her weight checks every single day and she’s been dropping pounds like it’s nothing. I don’t know how much you naturally lose so I don’t know if it’s an alarming amount or not. Then things got really odd. Sometimes we go down to the local shops to get food together. She will never get her own. She’ll just take one chip from my mate and that’ll be it for the night. Not too long ago, she was panicking over chewing gum because she hadn’t checked how many calories were in it. She only will drink certain drinks and will always check the back first. I thought it was kind of worrying but I don’t know her that well so I figured maybe she was just picky or something. So I asked my mate, her boyfriend, about it and he didn’t seem too concerned and said that’s just how she is. Everyone that is mutual friends with me and her don’t say anything about it either. I thought maybe it was something serious but the fact that NOBODY else is reacting at all is making me feel like I’m overreacting. Could I get some insight?