I could have sworn I saw this somewhere recently, but can’t find it.
Let’s say an ally is using Cartographer to map something and I give him a helping die by using my Scholar skill and then spend a check. Do I earn advancement for Cartographer or Scholar?
As I read the rule, you “log a test for advancement for that roll”, so it is the active character’s skill that counts. In your example, you help with Scholar but log a success/failure for Cartographer.
This makes sense to me, as it means the helper is learning from a character who is stronger in that skill. It makes it a bit easier to log successes in skills you are weaker in, allowing you to advance your skill rank if you help someone who is better at it.