"Learning" as a noun

That is ill-informed nonsense. Of course it’s a noun. Point the tweeter in the direction of Alexander Pope’s couplet, both for the usage and the sentiment:

A little learning is a dangerous thing;

Drink deep, or taste not the Pierian spring.

It is not entirely clear whether the tweeter had the plural form particularly in mind, but, if so, its use is not unprecedented. In Cymbeline, I. i. 43 the First Gentleman says:

The king . . . Puts to him all the Learnings that his time Could make him the receiuer of.

Francis Bacon uses it at least twice:

Particular and positive learnings of lawes doe easily decline from a good temper of iustice.

and

He did send his diuine truth into the world, wayted on with other Learnings.

It's perfectly correct English. "Learning" is a gerund, a verbal noun:

MW3UDE: Main Entry:learning Function:noun Inflected Form:-s Usage:often attributive Etymology:Middle English lerning, from Old English leornung, from leornian to learn + -ung -ing

1 a (1) : the act or experience of one that learns the learning of a trade *gives T evidence of trial-and-error learning in paramecia— W.N.Kellogg* learning may be regarded as a property of all living organisms— R.C.Noble *learning experiences* (2) : the process of acquisition and extinction of modifications in existing knowledge, skills, habits, or action tendencies in a motivated organism through experience, practice, or exercise — compare MATURATION b (1) : something that is learned or taught increasing the practical value of the learnings— H.R.Douglass *the film does provide learnings— Catherine M. Adler*; specifically : a subject that is taught in school emphasize the mastery of essential learnings— M.B.Smith (2) obsolete : ACQUIREMENT

2 a : knowledge or skill acquired by instruction or study : ERUDITION book learning *a man of good education and learning— Jonathan Swift* obtuseness in perception can never be made good by any amount of learning— John Dewey b : knowledge accumulated and handed down by generations of scholars : CULTURE learning is a sacred deposit from the experience of ages— William Hazlitt *Assyrian learning of the seventh century B.C. is well represented— H.J.J.Winter*

3 dialect : formal education : SCHOOLING synonyms see KNOWLEDGE

Your friend is probably fed up with learning. According to OED and NOAD, learning is a perfectly normal noun.

learning |ˈlərniNG|
noun
the acquisition of knowledge or skills through experience, practice, or study, or by being taught: these children experienced difficulties in learning | [ as modifier ] : an important learning process.
• knowledge acquired in this way: I liked to parade my learning in front of my sisters.
ORIGIN Old English leornung (see learn,-ing1).