I would ask her whether she would like a big ring with high quality or pay the huge mark up for a smaller ring but Tiffany brand. If you buy from Christies or Sothebys auction houses, you know you are buying quality as it often includes the GIA report and mentions color, clarity, etc. A high quality is D or E color and cut at least a VS2.
Used to feel like you did. Was at their auctions in December. Most of the stone were over 7 cts & no bargains to be had. They now have a 25% buyers fee which doesn't help. If you are looking for a 30 ct D IF (they actually had one!), you'd probably do better here than Graff.
Diamonds with 'Tiffany standards' can be found many places.