There is so much to check over when purchasing a Luger, both for collectors and for safety purposes that it can't possibly be set down in a posting here.
They have been mfg'rd by various makers, for multitudes of Counties and for the commercial markets all over the world.
Markings, proofs, finishes, ser#'s and all sorts of things effect values. Many Lugers have mixed parts from other pistols and not necessarily from the same era either. Everything vastly effects value and some smaller issues are not readily seen w/o some experience with the Luger.
There are probably dozens if not more WW2 variations alone,,each with their own set of correct collector specs. Anything less diminishes value,,the trick is to know what and how much.
Some variations are common,,some very rare though they may look the same to the casual person like your friend who 'is not a gun guy'.

A shooter grade 'mis matched' ser#'d WW2 issue Luger could be a $500/600 gun depending on bore and finish.
It could easily be a $2500+ gun with matching #'s, excellent bore and orig finish.
Might be a Kreighoff mfg pistol instead of a Mauser,,but you have to know what you're looking at.

Not a good one to learn on by going in blind.
Do the homework first before buying.
There's some real junk & refab'd Lugers out there too that aren't worth the $$