That largely depends on the lighting and what else lives in the tank.
Most easy and hardy are soft corals, mushrooms, polyps, zoas/palys, green star coral.
A little more involved are large polyp stony corals, frogspawn, duncan, hammer.
Well what type of tank is it now ? And I'll ask first cause ur gonna get asked to list all parameters, lighting, any info about the tank u can provide will help
We if all pareters are good I would tell her to start off with either softies or some lps beginner corals and see how they do try a frogspawn it's a beginner but really pretty
Candy canes are easy to care for and not expensive.
Frogspawns and hammers are also an easy group to have and you have the nice wavy motion they make.
Open brains are also easy to care for but they should be fed and they are a little more costly but the colors are amazing. The feeding tentacles come out about an hour after the ligts are out. They need to be fed finely chopped foods. They will take bigger pieces but chances are they really won't digest them.
In a tank that size I would go for colonies which show up nice. You could start with frags which is cheaper and grow them into colonies.