His concern could be allayed by buying the ring she loves and adding a personal touch, like an engraving, and choosing to see the issue differently.

So you get to tell him he should feel different, but you don't think there's any room for her to feel different? I'm done here. You don't seem to be getting how one-sided you are looking at the issue.

she also has a practical concern on top of the emotional concern.

What's practical about a solitaire compared to a halo? A solid band to a pave? I back her up on wanting a low profile, but there's so many different ways to get a ring that shape. They wouldn't really feel much different. It's about appearance, not practicability.