She is thinking solitaire because she has no imagination over how distinctive a ring can be.
The term solitaire simply refers to a ring with a single stone.
That says nothing about design- most solitaire settings have a low band with TALL prongs holding the stone up vertical like a doorknob.
THIS is the kind of ring she will have to remove to do anything practical.
However channel set stones do not stick out at all- are more secure, and may result in a ring that she can wear daily.
If you want to go diamond, you might want to go for a VERY expensive and rare colored diamond, in which case a very small stone, that is very nearly perfect might work in your favor as it frees up design options and may result in a ring she can wear 24/7.
Attached are some pictures to give you a very rough idea of the scope of what you could go for... and this was just a cursory search online... Custom designed jewelry can get WAY more unusual and distinctive than these...
And can just as effectively do something unique and special in a very understated way, if that is the kind of woman she is.
One of these looks like it is made of wood- its not- its two kinds of gold that are forged together like damascus steel- creating that pattern, which is called a damask.
The one with green/blue accents is actually titanium, and shows how a small, very expensive stone can feature into an interesting result.
And one of these just shows how elaborate and distinctive you can get...
The multiple stones could be multiple colors , or multiple sizes... could describe the constellation of her birth sign, etc.
Start by searching online... and, again, think about her personal style... her colors, her job... and pick something that will surprise her with the thoughtfulness you brought to it.