Into the Wardrobe

The Four Loves has good explanations of Friendship, both male and female. I can't give you any exact quotes, but there's some good stuff there.
I don't agree with Lewis' comments there on female friendships, but I can excuse him considering the type of women he was exposed to every day.

I remember him saying in The Four Loves that friendship is based on mutual interest. A friend is someone with whom you have interests, or likings, or hobbies, in common.

He also mentions that Cicero said this was the highest form of love. We, in our post-Fruedian society, shy away from talking about same-sex friendships, deeming them all rooted in homosexual attraction (even if it is way down deep, they say, it's there), but I think friendship is a notion we ought to we revive and value as it was in the past.

A#Minor is right, The Four Loves has a lot to say about friendship -- too many lines to quote just one, although I like, "Do you share the same truth? Do you care about the same truth?"

In practical terms, looking at Lewis's actual friendships tells us much about the value he placed on his male friends. There's the aching pathos in an aged Lewis's final written line to perhaps his closest friend, Arthur Greeves: "But oh Arthur, never to see you again!..."

Lewis had some deep friendship in his life. His poem, "On the Death of Charles Williams," has these lines. After he talks about how animals can make friends and be with others of their species without problems, he then says

Not so is man, who in his creatures find
(Hard fate) discordent souls and alien minds.
To him, though searching hard, will scarce be shown
One soul within a thousand like his own.
Or if, at last relenting, fate shall send,
In answer to his prayer, the authentic friend,

then he goes on to say the friend will die "and leave behind a loss that time cannot allay."

What Lewis calls "the authentic friend" is a rarity for most people. I know it has been for me.