At a prior company I flew with CA's who'd never seen a strong crosswind before, as they were new to the type. What would you do in that situation?
I fly for the same airline as Penko (>200 aircraft as it happens). the situation you've given will not happen very often: there are a few non-rated DECs but not many, so in the majority of cases the captain is going to be the more experienced on type. However, in your situation, there is still no discussion. Rightly or wrongly (and I would personally say wrongly) any landing with more than 20kts crosswind is the captain's, with an alleviation for training flights.
If the first officer is considered legally as second in command then presumably he would be certified competent at the end of his type rating to take off and land up to the aircraft limiting crosswind component. After all he has a command type rating - not a co-pilot type rating.
It's a while now since I did my Airbus type rating, but I'm 100% sure that maximum crosswind landing and takeoff were not part of the syllabus. Looking at the syllabus for the company type rating course, maximum crosswind landings are not required here either.
That suggests to me that it was not required under JAR/EU OPS and is not required under EASA.