This is often done with people or abstract things (things you can't touch).
They are really nouns but are declined like adjectives. And since it is a noun, it must be capitalized.
Normal sentence: Article + Adjective/Participle + Noun
When the noun is obvious, we can simply remove the noun and capitalize the adjective/participle (since it's now a noun!)
In this case (because of the context of the situation), "Freund" is obvious and can therefore be left out. The adjective "neuen" becomes a noun and is capitalized. But we still have to keep the adjective ending.
Nouns that are often obvious and therefore left out include "Mensch," "Mann," and "Frau."
Participles are often used as adjectives. Just like other adjectives, we can remove the noun (if it’s obvious) and turn the participle into a noun by itself.
All of the endings in these examples are in the nominative case. If the noun is in a different case, it would have a different ending.
An overview of what you have to pay attention to when dealing with adjectives in general.
A detailed explanation of the rules for Adjective Declension.
A simple step-by-step guide for finding the right adjective ending: Adjective Declension: 4-Step Rule.
Adjective declension depends on whether there is a definite article, indefinite article (including possessive determiners or the negative article), or no article at all (the zero article) in front of the adjective.
You have to know the gender of the noun.