Today, start keeping a food journal. Use a notebook (I mean a paper one) with one page for each day. Note the time of any food and drinks, and note the food or beverage that was consumed. And be specific. Don't write "afternoon, had a snack". Write "3:00 pm. Had three Oreos." Or "8:00 pm, drank 8 ounces of 100% pure orange juice and ate 2 pieces of toast - white bread - with Parkay margarine and grape jelly". That kind of thing.
See how much water she is drinking during the day. Don't say "she had 2 glasses of water." A "glass of water" can be a shot glass or a huge 16 ounce cup. For awhile, buy bottles of water and have her keep track of how many she's had. Tell her not to throw the bottles away but to put them on a counter top, and at the end of the day evaluate how many bottles she drank. And make sure she's not dumping any of the water out. If she doesn't finish a bottle, put the half-empty bottle on the counter. Write down the ounces in the notebook. She may be drinking excess amounts of water or not nearly enough.
If she takes any medications for any reason, talk to the pharmacist about side effects. Ask the pharmacist when she should be taking these medications and how (with food, without, spaced apart, etc) and make sure that is being followed to the letter.
If she takes vitamins, or supplements, or makes smoothies from powders, evaluate all the ingredients and the source. Look up the ingredients.
Evaluate any over-the-counter medications you're using to help curb the nausea and headaches. Talk to the pharmacist about doses, and proper usage.
Has she had an exam by a doctor - a gastroenterologist, a headache specialist? Has she had her blood and urine tested for electrolytes, vitamin deficiences, dehydration, etc? Does she vomit or just feel nauseated? How long do the symptoms last? How is her sleep? Does she go to sleep without any electronic screens (no phones, tablets, tv), and are her bed and mattress comfortable?
Tell us what the situation is, and what you've tried, and what doctors she has seen.