There's some interesting discussion on ghosts and teaching in this old thread:
https://forum.atlas-games.com/t/ghosts-teaching-the-living/348/1
Note that this predates TMRE and its Living Ghosts which Timothy details above and also the Summoning/Commanding rules from RoP:tI. This latter source allows the use of Commanding twice a day for a season to force an entity (which could be a ghost IMO), to Teach the summoner one Ability it knows. YMMV.
I can't see the problem with commanding a ghost to teach you, although using the precedent of RoP:tI, it requires sufficient proficiency and time to be able to command a ghost for the whole season ie. 240 separate rolls in order to get the usual Teaching Source Quality, otherwise it's just Adventure Xp (RoP:tI, page 118).
I hope for your magus's sake that he can command a ghost without using stress dice as the statistical chance of even one significant botch is enough to give pause...
Regards,
Jarkman