There are many causes of heartburn. Your LES can be weakened. You have have too much, or more commonly too little stomach acid. You can have enzyme deficiences, or food allergies or quite commonly its SIBO - bacteria in the small intestine.
Is there any sugar in her salad dressing? That could be the culprit (most dressings are high in sugar, sugar feeds bacteria in SIBO)
For everyone reacting to eggs, nuts or grains -
Eggs whites, along with nuts and grains all contain some anti-nutrients - these are to protect the vulnerable seeds and eggs from predators by making them hard to digest and problematic to eat.
People with compromised digestion, particularly leaky gut caused by SIBO or other diseases can develop intolerances to anti-nutrients - although usually grains are the worst. Anti-nutrients when they are absorbed through the mucosa of the gut over time produce these food allergies.
Its worth noting that the egg yolk does not have anti-nutrients, and also contains all the good nutrients..
Hope that helps..
For the question poser - i would suggest she tries both a sugar free dressing, and eating an egg yolk without the whites.....and then look into the causes for her heartburn based on that (bacteria, or leaky gut/anti-nutrient intolerance)
Of course id just as likely suspect the sugar in the salad too as it could also be SIBO.
"A liquid antacid will provide prompt relief."
Your post is very confusing. An antacid is an alkaline substance. You cant make an alkaline substance less alkaline by adding another alkaline. Thats like putting out a fire by throwing some fire at it.