Is friends a common noun or a collective noun?

The noun 'friends' is a common noun (unless you are referring to Quakers, in which case it is a proper noun), a word for people.

A collective noun is a word used to group people or things taken together as a whole, for example a group of friends or a bunch of friends.

All collective nouns are common nouns, but not all common nouns can function as collective nouns.