Ben Sira on Friendship

When you gain a friend, gain him through testing, and do not trust him hastily. For there is a friend who is such at his own convenience, but will not stand by you in your day of trouble.  And there is a friend who changes into an enemy, and will disclose a quarrel to your disgrace. And there is a friend who is a table companion, but will not stand by you in your day of trouble. (Sir. 6:7-10)

And

With friend or foe do not report it, and unless it would be a sin for you, do not disclose it; for some one has heard you and watched you, and when the time comes he will hate you. (Sir. 19:8-9)

And

Every friend will say, “I too am a friend”; but some friends are friends only in name. Is it not a grief to the death when a companion and friend turns to enmity?  (Sir. 37:1-2)

And in each instance, he is correct.  Many who call themselves friends really aren’t and though it’s sad it’s best not to open your heart to any but family, because even those you think friends will betray you and, accordingly, wound you.