I'm not a jewellery girl at all, but let me tell you, I look at the ring regularly (and this is 14 years later) and say out loud "Somebody loves me." Plus the ring is really spectacular and exactly the style I wanted. I actually wear my ring all the time, even when I'm sleeping and swimming.
I like the idea of presenting a pearl ring and then shopping for another ring together. The pearl necklace would also be great.
(But I do hope you get her a diamond ring if that's what she wants rather than just a band to match yours. If you don't offer a diamond, she won't ask for one. I also like the idea of asking a friend--maybe ask the friend to ask her if she wants a diamond. Even if she suspects that you put her friend up to it, she won't know for sure and she will feel free to speak the truth.)