I do not know the exact reason you/she are omitting the gelatin, but I just wanted to let you know that marshmallows contain gelatin
Marshmallow Fluff doesnt contain gelatin but it does contain a form of egg whites, so depending on your dietary restrictions and your reasons, you should be aware of this.
For fondant recipes that call for gelatin I would substitute agar (it needs to be cooked first to dissolve the agar and get gelled before you can use it). Substitute powdered agar for gelatin 1:1 (1 Tbsp of agar flakes is equal to 1 tsp of agar powder). If you can only find flakes (that's what I usually use) you can also use a grinder to turn the agar flakes into powder so you don't need to convert amounts, if you prefer. You can find agar in health food stores (they usually keep it with the seaweed) or online.
Alternatively, you could try a recipe for fondant that doesn't call for gelatin. I found this one online a while back but haven't gotten around to trying it yet:
2 cups raw sugar
1 cup water
1/8th tsp cream of tartar
Using candy method bring ingredients to 238F. Finish as per fondant method.
You knead in any flavouring or colouring you want when the fondant is cooled and almost pure white. You can freeze this fondant for up to 3 months (let it thaw and re-knead it before rolling).
If you're new to candy making they recommend replacing 1/4th cup of the sugar with 1/4th cup corn syrup to help prevent crystallization (which wrecks the fondant). They also refer to this website for detailed instructions about how to make fondant (where there is another gelatin-free fondant recipe you might try):