Extracting files from partitioned database
As I understand it, following the merge each partition will have its
own mdf and ldf files and these will contain all the data for one
customer.
...
My question is, given that I can get the files for a single customer
from the database server is it possible to copy them to my development
SQL Server and create a database for that single customer?

That is wrong.
There are no &quot;partitioned databases&quot; in SQL Server, there are partitioned tables and partitioned views. I suppose you tell us about partitioned tables.
Normally, every database has ONLY ONE log file, and even if for some reason some database has more than 1 log file, it has nothing to do with partitioning. So one more time, you misunderstand partitioning.
Partitions are NOT independent databases within your database, you cannot get out one partition by just copying the corrsponding .ndf file. You never can attach a single ndf file, it will never succeed.
And even if you'll try to make partial backup/restore you cannot restore database from one ndf backup without restoring PRIMARY filegroup