Maybe once he proposes, he could show it to her and ask her if she'd like to wear it and explain the significance of the ring.

Personally, I don't wear any rings other than the anniversary band that was my mothers. My husband is ok with it.
(I don't even wear it very often, since I use my hands doing data entry all day, my fingers feel weird with jewelry...)

And another thought - someone else mentioned this, but engagement rings don't have to be only diamonds. Look at the "Princess Diana" ring that Catherine is wearing now. It's a Sapphire. (And gorgeous!)