I did custom for two different sets of stores. The first one just used the large ads provided by the vendors. . But... in the second one, we created fun funky seasonal window displays. Sometimes they tied to art exhibits currently getting a lot of press; sometimes it was something surreal like vintage porcelain doll heads (for Halloween) that we borrowed from a local antique shop. One of the most talked about window displays we did were rubber chickens... the silliest display but people loved it; they came in just to ask about it... so you never know what will get the best response but to create windows that people talk about generates free buzz.
If I were your friend, I would create two smaller display cases in the widows - it helps direct the client's eye. At night, put up photos or a poster. The jewelry within each case should compliment each other but each case should be different (i.e., one is fashion and one is bridal). I believe a successful window display is part art and part product.
Tell your friend to be adventurous; think outside the box and check local resources for "props" (give them credit so they have an incentive to contribute). The windows reflect the personality of the store but from Tiffany & Co to the regional stores I designed for - creative windows always generate business.
Finally, if people can't tell it is a jewelry store when they drive by - that's a lot of lost potential clients. Awning or better signage would be my first change.