Hi J,
What a nice collection your friend has, a wardrobe of pearls already!
Since the Tahitians are pretty dressy (probably) and she has the earrings and pendant,
Why not a bracelet? You could choose baroque Tahitians with a green overtone, then add to it a special charm that would have meaning for the two of you, or an initial pendant for her name, something like that. It would make it more personal. Being an engineer type, she might prefer rounds over baroques, but the colors on the baroques are so often spectacular, one falls in love in spite of preconceived notions about what one likes.
Or--a strand of graduated white pearls. A bit more unusual and harder to find. Very traditional, but totally wearable with the most basic sweater or blouse.
Always consider quality! However, with all the lovely freshwaters available, there are lots of high quality available without totally ruining the budget.
My first pearl was a gift from my "going steady" boyfriend, a single white pearl pendant on silver chain, back in the late 50's
And no, I don't have it any longer.
A fun subject always--pearls, and more pearls, and more pearls--
Pattye
so many pearls, so little time