Major Korean department stores all have a wide variety of Korean teas
on sale in the tea sections of their food departments (invariably one
of the basement level floors). Assuming your friend is visiting Seoul,
she could explore the Lotte Department Store in Myong Dong (central
Seoul and the city's main shopping area).
Many Korean green teas are similar to Japanese green teas, and some are
made from Japanese tea plants and processed Japanese-style.
However you can also find exquisite hand-made teas produced using
traditional techniques in very old tea gardens that are part of
Buddhist temples. These can be very expensive, partly because the
volumes made are quite small - but they are much closer to Korea's tea
making traditions than many other offerings.
There are also inexpensive blends of green tea with barley or other
grains - these are often imported from China.

David