I realise rings are a highly personal choice, different budgets, and to each their own... but bleh I think that stack looks like a complete cluster. If she wants a flash and firey situation on her left finger the answer is a 4-carat brilliant-cut solitaire.
If I were her I would ditch the crown rings and get a different pave eternity band with less metal (current one a quite heavy), She could also change the band on her engagement ring to pave diamonds if she really wants that sparkle.
Working with what she's got a best case scenario is a set like this:
http://weddbook.com/media/2426910/p...-wedding-set-wedding-band-and-ring-by-zhedora
Her engagement is "small" (looks like .75-1 carat?) relative to what you expect from her, but it is Tiffany, we know it's an amazing cut and setting.
It's hard to know the colour and clarity without seeing it, but let's assume they're nice as well.
I think the above would be clean, crisp, classic and enhance rather than the current drowning situation while providing a bit more sparkle and shine..