OH my, loved your answer Librasoul. Good for you girl!
And I think Justamom may have hit the nail on the head. The engagement ring is something showed off, and obviously she doesn't want to show off what she considers an inferior ring.
Is that shallow or normal?
And Valkyrie with the surprisingly girly answer that the ring should be financially more significant. A surprise!
Now for the true fact: She is screwed!
She snooped on his computer. It takes a bit of effort to go to a book mark. She came to a conclusion and now she is upset. If she is like most people she will sit there and dwell on it making herself unhappy and putting a cancer in her relationship.
This is no small thing to her. This is forever after! I love it! She and he will suffer because she went where she shouldn't have and jumped to a conclusion without asking him.
If she wants to salvage anything she should probably make an innocent comment about finding a link on the computer . . . or immediately show him some magazine with the type of rings she actually likes (as well as their price tags ;p) otherwise this might be a real problem later, because as others have pointed out, there are a lot of assumptions here.
PS. If a girl I was dating seriously reacted this way. I would drop her. Even if it was a misunderstanding, and that wasn't the ring I was looking at for her.