Need advice, girlfriend going from eating 500 cal a day to 2300

[salminbel28]

Hello, my girlfriend has had knee surgery multiple times and went to being quite athletic, to now being on the skinny side, but not underweight. She appears completely healthy and is a good weight, but is now getting back into fitness after a long hiatus. I'm personally into fitness/nutrition, but everything I know is just personal experience from the past 5 years. I don't want to be giving her advice when it's possible the things that have worked for me, won't apply to her (being different sexes, having different levels of athleticism, etc).

She says she eats like 500 calories a day normally, and now her sister who is into powerlifting has her eating 2300 calories a day. This seems like a huge jump for someone who possibly is suffering from anorexia. She says eating that much food makes her feel like she's going to throw up, etc. I want to help her increase her weight(her desire) safely.

Should her main goal just be eating more food or should she start tracking all of her macros and be precise with it? To have an 1800 calorie a day jump seems like you're bound to have your body go into panic mode and start storing lots of fat, but again I have no formal training in the nutrition world, so maybe I'm totally wrong. I track all of my macros daily and she has started to as well(her desire), it works great for me, but again, I'm not trying to put too much pressure on her and her diet, especially for someone who has issues with food. Could use some advice, I feel like most medical doctors advice would be pretty outdated old-school stuff they learned during their residency as most MDs don't do a whole lot of nutrition training. Also, most specialized dieticians are not covered by insurance, and/or still use a lot of outdated information from the 90s.