You probably wouldn't be friends with a back stabber, but would you be friends with a front stabber?

I have been friends with "Penelope" & her family for 30 years. She & her husband now live out of state. I occasionally go to lunch or shopping with Penelope's mother, "Gladys". Recently, we went to an estate sale together. I saw an antique mirror that I really wanted to buy. I said to Gladys, "Ooh, look at that mirror! I love it! I wonder how much it is?" Gladys said, "There's the owner. Ask her!" The owner said she wanted $45 for the mirror. I was very disappointed & told Gladys that I would pay $20 or $25, but $45 was just too much. Gladys agreed. I went outside to wait while Gladys paid for the items she was buying. When she came out, she was very excited to tell me that the owner's husband sold her the mirror for $15! I was thrilled!! I thought that my "friend" helped me get the mirror I wanted! It wasn't until we were in the car that I realized she didn't score the bargain for me, but for herself! She said how funny it was that the owner's husband sold her the mirror for $15, right after his wife told me it was $45! I was shocked when she told me she was going to keep it! I didn't say much because I was so shocked & she is my elder. She knew perfectly well that I wanted the mirror. Eventually, she said that if it didn't look good in her bathroom that she would sell it to me. I was hurt & disappointed, but wanted to believe that Gladys was just insensitive & oblivious.That week, Penelope came to visit & I told her what had happened. She just laughed it off & said, "It's just a mirror!" That same day, I was at their house & saw the mirror leaning against a wall. I asked Penelope what her mom decided to do with the mirror. She laughed & said, "She gave it to me! Isn't that funny!" I couldn't believe it! This has nothing to do with the mirror! It has everything to do with their character! When I tried to talk to them about it, they just wave me off & say, "It's just a mirror!" How can I remain friends with people who are so thoughtless & selfish?