I feel bad for your friend to quote from a movie "demon's exist whether you believe in them or not" and trying to keep people from seeing or knowing about them isn't helping, and TBH Sunset's demon form isn't even THAT horrifying or scary, maybe to a degree for children 7 or under. (I watch anime and horror movies I've seen worse) but your friend does have the internet and they can use it to expand their knowledge and make themselves aware of things that are good and are bad and if they are religious know this, God will always be mightier then any demon so if their parents are afraid they will be traumatized they should just tell them that God is with all of us and protects us. heck even the powers and elements of harmony that give strength to Twilight and her friends are in a way based off of the virtues that religion tries to teach Apple Jack is Honesty "thou shalt not bare false witness against thy neighbor" Rarity is Generosity "Thou shalt not covet thy neighbors wife or property", Fluttershy is Kindness (Love thy neighbor as you love thy self) etc. tell your friend these things if you get the chance, I hope this might help and I apologize if this was a bit long and preachy
Probably not. According to fan theories, they are probably around 4 feet (with the exception of Celestia and Luna). In terms of artistic balance, maybe she was made small to focus on the human forms in this piece.