I think part of the problem is that people are way too shallow. This girl probably saw MTV's Wedding Story or one of many other shows where people spend 5K on the cake alone....
All I can say is that those are not real people. They just have more money than sense. However, it seems that the impressionable audiences that watch those shows think that if they don't have a five thousand dollar cake and a ring that costs $15k, they will be shamed in front of all their friends.
Obviously, as it has already been pointed out, that if the girl is questioning her relationship because the price of a ring there are other problems. I know that I could never marry someone that is that shallow and superficial. I would almost guarente that this isn't an isolated incident-there are probably many other situations in life where her insecurities will crop up.