Just a warning that bringing plants and plant parts into the US is very difficult unless you have the proper certificates, etc., but perhaps someone else can be of more help.

Korea has four seasons like most parts of US, but each season is very distinct and quite clearly marked. Hence, comparing latitute may not be a good for identifying the zone.

As peaseblossom says, your friend should be filing for certificates to properly bring plants or seeds into the US. If you're interested in growing Korean varieties, prolly better off shopping from US companies who import and sell them here - these include Evergreenseeds.com and Edible landscaping for fruit trees.