I will also visit Java someday.India is so good at technologies. I admire it.
which part of Java?
if your friend wants
metropolis: Jakarta, Bandung, Surabaya (my hometown)
mountain: Bromo in East Java province (my province) because there's a Hindu tribe called Tengger that inhabits the Bromo-Tengger-Semeru mountain. Semeru whose history is related to Indian history (once called Mahameru)
temple: Borobudur (biggest Buddhist temple in southern hemisphere), Prambanan (one of world's biggest Hindu temples), Trowulan (a small town where Mojopahit was located
beach: Pangandaran, Parangtritis, Prigi, Popoh
and many more. Java is a wonderful island