How to tell a friend/roommate..

Abby
My friend and her boyfriend are our roommates and constantly fight over the smallest and most stupidest thing. Happens an average of 20 times a day! And I feel as if they try to one-up my boyfriend and me. They don't have well paying jobs and spend more money than they can make. My boyfriend has a very good and very well paying job. So for Christmas he bought me a pair of diamond earrings. He gave them to me a few days before hand because he wanted me to wear them for Christmas <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">eve</a> and day. As soon as my friend found out I received diamonds for Christmas, she immediately went and told her boyfriend she wanted diamonds as well. So he, in turn, went to get her a 'diamond' promise ring. I take the trash out every Tuesday night. As I was doing so, the bag from my friends room was so heavy it ripped. As I picked everything up I came across a Zales box. I stopped bc that's where my earrings came from and I had this gut feeling something wasn't right. I opened the box and found a receipt from Wal-Mart for a ring for $47.23 dated 12/23/12 at 1:24 am. I checked for my own Zales box and found it was missing, and realized that my friends boyfriend went into my room and got it to make the promise ring look like the real deal. Later on the next day, she comes up to me and shows me the ring. Sayin your not the only one who got diamonds this year. She said it was a 24k gold band with a 1c diamond. I said it's pretty and walked away. How do I tell her it's fake?