Go shopping wherever locally to see what style she wants
Buy it online. Lab grown diamond. They're indistinguishable from blood diamonds except for the price. And if she's snooty enough to need a blood diamond he'll be able to dodge a bullet. Can get a completely custom setting with a good sized, very clear, highly graded diamond for that price.
I just bought one last month from Cullen Jewelry out of Australia and 1000% recommend them. You can pick the exact diamond, setting, etc. A real person will send you updates as they make it, send you a video of the final ring for approval before they ship.
(She said yes)