a friend is someone with whom you have a bond of mutual affection, respect, trust, and support.
rules and use cases for when the word 'friend' is used as a noun includes that it is always used in the singular form unless using a plural possessive pronoun (e.g. 'my friends' house') or when discussing several friends in which case it is used plural (e.g. 'My friends and I went to the park').
1. She is one of my closest friends.
2. My friends and I had a sleepover last night.
3. I consider him a trusted friend.
The word 'friend' should always be used in the singular form when talking about one person. Also, keep in mind that it is generally used in a positive sense; it should not necessarily be used to refer to someone who you don't personally know very well.