A few months ago, I happened to reconnect with one of my friends, and noticed she was thinner than I remember. She had always been a thin girl, but now she looked even more so. Just yesterday I saw her again this time looking even slimmer to the point I felt concerned. I pressed the point a little and she revealed she now weighs only 86 pounds on her 5'1" or so frame (~155cm and 39kg). I tried to light-heartedly suggest she eat more, but she deflected and told me I should lose weight too (I'm 5'9" 160 lbs – 175 cm and ~73 kg – so a tad chubby but nothing remarkable).
I'm worried she might have an eating disorder and I want to help, but I don't know how to constructively engage with her. I don't want to appear as overbearing or mansplaining to her and I'm aware ultimately it's none of my business, but I'm concerned she's going to keep losing weight and hurt herself. Any help would be greatly appreciated.
EDIT: A few clarifications and more information based on comments. We are both in our late 20's. I met her when she was a research assistant in my city about 2 years ago. She is an international student, and just started a PhD program about 7 hour drive from where I live. I don't think she has any friends/support system in her new city and has minimal close friends in this country at all. Her family is also still in her home country; I'm unsure of their relationship. Either way I don't know enough about to get in touch with family/friends, and I believe I'm one of her closer friends on this side of the ocean.
She does seem to have at least mild health symptoms of being malnourished. I made a joke about telling time from my receding hairline, and she responded that she was losing her hair too in a much more serious tone. She stayed in my guest room and when I changed the sheets this morning I did notice she left more than a few strands on the pillows. She complained of being tired quite frequently during her visit. She also has noticeably sunken cheeks, and her ribs were visible through a thin tight fitting shirt she was wearing. She doesn't look like she's on death's door, but clearly looks underweight from a visual perspective. The weekend she spent with me she ate what I considered to be a smaller than normal portion (ie for dinner she ate ~6 chicken wings and a handful of berries and yogurt for dessert after having a small salad for lunch) but she didn't seem to be avoiding food or uncomfortable at meal time.