CHEARS

Actually lemon juice itself is citric and acidic in nature, so insufficient amount of food and with the intake of lemon juice can precipitate pain. So tell to her to drink plenty of Water and also ginger mint juice. And also if you have any hot air bag or bottle, place that in the stomach region.
If possible provide bananas since it is a natural antacid and also have potassium in it, which can reduce pain.

If the pain is persisting, Give paracetamol or acetaminophen.
If diarrhoea is occuring, Tell her to take loperamide .
Also pain is due to energy deficient, so take glucose or electrolytes and if possible carbs or bread.

Don't drink lemon juice or orange juice because it will increase acidity. Just drink normal biled and cooled water. If you have fenugreek seeds, you can just swollow it with water to prevent gastritis. If you are able to go out, please take rabeprazole + domperidone combination

Since she hasn't eaten anything for the last two days there is increased production of gastric juices , so when she had lemon water again the acidity increased hence increasing her pain...
I would suggest to give her lots of water , avoid anything acidic for a while.
Do you have access to medical supplies?
If so please give her the medicine- Pantoprazole 40 mg( 30 minutes before food morning and evening), it will help in decreasing acid secretion in her stomach .
Also avoid oily foods .