Lots of Peanut Butter, eggs, and if she can stomach protein the easiest way to put on weight is through Protein Mass Gainer. 600 calories plus 55 grams of protein in liquid form is a lot easier to take in high calories than healthy food options and much better than downing McDonalds and Wendys. It shouldn't replace a meal but if she can drink one down to supplement her meals it should make a big difference.
Making high calorie yet healthy small meals is extremely harm to do. Like Tuco said, find what she likes and can eat and then jazz it up with calories. For example, if she likes sweet potatoes, make drunken sweet potatoes instead so the butter can up the calories. Or green bean casserole instead of just having green beans. The problem though is these type of meals are more involved and probably harder on the stomach.