Can we use 'calm' as a noun?

What both you and your friend have done is made an adjective (hardcore, calm) into a noun through the process of nominalization. In this case, both of you use the adjective form to specify a group of people who have that quality: the hardcore (people), the calm (people). We see this in many established idioms and proverbs:

Blessed are the meek, for they shall inherit the Earth (Matthew 5:5)

How are the mighty fallen! (2 Samuel 1:19)

Both the meek and the mighty describe a group of people who have that quality. These usages are often not recorded in dictionaries; calm is does not appear as a noun referring to people in Merriam-Webster. However, if it describes a person, using an adjective with a definite article as a noun is one way to generalize about a group of people:

I believe only the calm survive.

The bold may win the day yet.

There are the quick and the dead.

One other thing to notice - all of these examples use plural concord, that is, the verb agrees with a plural noun. Saying "I believe only the calm survive" denotes an implied plural subject corresponding to the verb. (Example: "the calm people survive".) If you had said "I believe only the calm survives," then the verb would suggest a singular subject, and might refer to various singular meanings for calm.

The word "calm" can absolutely be a noun. The OED has several definitions that are nouns. These two are the most relevant to your example:

c. figurative of social or political conditions and circumstances.

1547 J. Harrison Exhort. Scottes 210 The stormes of this tempestious worlde, shall shortely come to a calme.

1609 Shakespeare Troilus & Cressida i. iii. 100 The vnitie and married calme of states.

1781 W. Cowper Friendship xxiii Religion should..make a calm of human life.

1850 J. C. Calhoun Wks. (1874) IV. 24 Till our free and popular institutions are succeeded by the calm of despotism.

d. figurative of the mind, feelings, or demeanour; = calmness n.

1609 Shakespeare Troilus & Cressida iv. i. 16 Our blouds are now in calme .

1719 D. Defoe Life Robinson Crusoe 234 All my Calm of Mind..seem'd to be suspended.

1807 Wordsworth Poems II. 108 A good Man's calm, A great Man's happiness.

1879 F. W. Farrar Life & Work St. Paul II. ix. xliii. 376 In that desperate crisis one man retained his calm and courage.