you shouldn't summon a demon just because your friend doesn't believe you, she probably won't be able to see it anyway. what you could do is cast a luck spell or something on her, mind you if someone doesn't believe in something, it's almost impossible to prove them otherwise.
try inviting her over to a circle. i had an athiest friend who, while he didn't believe in magic, he didn't stand around telling me why i was crazy and he wasn't. so he sat in on one circle, well my one coven member [really energetic Leo, and her element is fire] cast the circle. every time she cast the circle, regardless of where it is we cast it, the south [fire] candle always burns out in a matter of minutes. so, we feel a draft, and sure enough it burnt out. my covens pissed because we have to recast and find a new candle and blah-blah-blah, meanwhile my atheist friend is loosing his mind because he can't figure out why the other candles are perfectly fine and the fire candle burned out. later he asked me to preform a luck/protection spell on his bracelet just in case. not even a week later he calls me up to thank me because he had met 3 girls [got a date with each one] and as he was walking along, he just randomly stop walking because he felt odd, right in front of him about 30 seconds later, a car came flying out of of a one way street [going the wrong way] and would of hit him. call it what you will, but that convinced him. so try something like that, tell your friend you'll cast a luck spell or something for her if she comes to a circle. might not change things, but it's far better than summoning a demon.