Everyone has given you good advice. You'll be looking for a small signal, in the low to midrange.
I've found a few stud type earrings, and they hit well enough with today's high gain detectors. However, they fade fast as you lift your coil a few inches.
And they can be difficult to spot. I chased one around shin deep water once, for quite a while....

Use the mate earring, or one similar, to get an idea of the response you're looking for.
If it's a trashy yard, well.... You've got your hands full.
I'm my experience, an item is also rarely where the owner thinks it will be.
So, if the girl is sure she lost it in a particular spot, you'll be lucky if it turns out that way.
I normally have to ask several questions and get a feel for the activities surrounding the loss.
Then, I find the missing item where it "shouldnt" be.
"Wow... I wonder how it got there?" is usually what I hear.
Last one was a 1952 Georgia Tech class ring. It was a long way from the suspected location and signaled just like the brass end of a garden hose (which I had ALSO found in the same yard!)
On the plus side, you'll have a new yard to detect in and you must investigate all signals. Who knows what else will turn up?
After four years, you'll probably be in for a full blown search.
I'd bring a lunch.