I Want My Girlfriend To Gain Weight

My girlfriend wendy is too thin. She says she's not, that she's always been skinny except for when she was on antidepressants in the past. IDK, she's 5'3" and she says about 105 lbs (I'm going to pick up a scale next time I'm at Wally World.) I think she needs to put on about 10 pounds or so but she says she doesn't. I monitor her eating and she tends to eat an average amount (for a woman on the thin side) most of the time, but when stressed she backs off on eating a lot, saying her stomach hurts.
What I wanted to know is, is her weight healthy/normal, or is she too thin? Maybe I'm overreacting, her BMI is in the normal range. But when I hold her it's like holding a skeleton, and she's cold all the time. I can see her individual ribs, and when she sits on my lap, her butt feels really bony against my thighs. I worry she will waste away if she doesn't start eating more, but she gets really uncomfortable whenever I bring up her need to gain a little weight. I'm wondering if that's a red flag? She says she's never had an eating disorder (never been diagnosed with one is probably more the truth,) but I think it's a gray area. When stressed she doesn't eat hardly anything. Recently she moved in with me, and now she seems to have a bit more of an appetite due to being less stressed. Still, I'd like to see her fill out a little more- not much, just a bit. I just want her to be healthy.