It is reciprocal pronoun. https://ru.wikipedia.org/wiki/%D0%92%D0%B7%D0%B0%D0%B8%D0%BC%D0%BD%D1%8B%D0%B5_%D0%BC%D0%B5%D1%81%D1%82%D0%BE%D0%B8%D0%BC%D0%B5%D0%BD%D0%B8%D1%8F

No, it isn't a noun, this is a set phrase which consists of two nouns. When it's said in a sentence, the first "друг"doesn't decline, but the second "друга" does. Мы знаем друг друга.- We know each other. Мы поможем друг другу. - We'll help each other. Мы довольны друг другом.- We are pleased with each other. Мы скучаем друг о друге. - We are missing each other.

It is reciprocal pronoun if you mean "each other". Sometimes it may be understood as "a friend of my friend". They are two nouns then.