well technology wise:
you can point them to the sutton hoo sword to start with and metion that in the 16th cent, the japanesses imported protuguess/spannish cuirrace (nanban dô)
Have a look
http://www.sengokudaimyo.com/katchu/katchu.html for more details
By the 1300 European were able to produce crucible steel hence a steel with the same or better quality that pattern welded/folded style with much less time and material needed.
Technique wise:
Well a katana/tachi is a longsword (well a shortish one) and you can find similarity in several Japanese school and European swordsmanship tradition (especially pre- Tokugawa)
As far as half-swording well that strikingly (excuse the punt) like Jo-jutsu.
If you take ringen, well people wo are doing ju-jitsu will find striking resemblance (and so are people who do judo, hapkido or aikido as well as people who are doing army combative or MMA)
If you want a concrete exeample use the bone breaker in Ringeck ringen.
The 4 at the arm are shiho-nague and the two at the left are used as defense against kick in aikido but it does exist in ju-jitsu definitely. (And I am pretty sure that other poster who do different martial art can give other example)
At the end European and samurai have two-leg two arms, a truck and one head (at least at the start of their career) and all that happen to work a similar fashion across the world so since they faced the same problems…..
One Ringeck to bring them all In the Land of Windsor where phlip phlop live.