Demographics are different.
If OP’s nephew wants a demographic area similar to current location of Spring Lake, NJ, then Falls Church may be the best option.
If one prefers more diversity, then Alexandria is a great option as it is much more racially diverse than Falls Church and more diverse than Arlington.
All 3 are wealthy communities.
Within Wash Dc, I like AU Park and Northwest DC because it is safe, upscale, and vibrant.