How do you say in french he is a friend of mine?

There are a few ways to say "He is a friend of mine" in French, depending on the level of formality and emphasis you want to convey:

Formal:

* C'est un ami à moi. (This is a friend of mine.)

* Il est un ami à moi. (He is a friend of mine.)

Informal:

* C'est mon ami. (He is my friend.)

* Il est mon ami. (He is my friend.)

More casual:

* C'est un pote à moi. (He's a buddy of mine.)

* Il est un pote à moi. (He's a buddy of mine.)

The best choice depends on the context and your relationship with the person you're talking to.