Cergy and Pontoise seem to be suburbs north of Paris, according to Google maps, unless there's more than one. There should be a number of dojos in and around Paris (Aikido is pretty popular in France). I was near there last year and quite enjoyed practicing at the Cercle Tissier http://www.christiantissier.com/cercle_tissier.html
, Christian Tissier's dojo in Vincennes (a suburb to the east of Paris). I also attended a (IMHO) great seminar with this guy http://www.aikido-benezi.com/dojo.html
. Of course YMMV, different people look for very different things in Aikido!
I found when I visited France that even very small towns often had Aikido dojos. Of course like anywhere there are different styles and teachers and training environments and quality, so sometimes it ends up being worthwhile to travel a bit further to find a dojo. But I'm sure your student will have no problem finding a dojo, his problem will more likely be choosing which one or ones to train at.
Luckily the trains are pretty good there, at least the Paris area.