I'm currently upgrading mine.
Although my circumstances are different. I moved and didnt fly for about 9 months and then started flying from Cranfield which is far far busier than where I got my NPPL at. Anyway i've done probably around 10hrs or so with an instructor (and some solo time) to get myself back into it but from my understanding once you've done abit of a refresh of things (stalling, etc) you can then do the general flying test.
However, for the NPPL I think the qualifying cross country is 100 miles whereas the normal PPL is 150. I did the 150 on my NPPL course and therefore dont have to do that when upgrading. However should your friend only do the 100 mile qualifying cross country he will then have to do the 150 one too when upgrading.
If there is anything else im sure someone else will add the info here.