Sirach 6:5-17
A pleasant voice multiplies friends, and a gracious tongue multiplies courtesies. 
Let those that are at peace with you be many, but let your advisers be one in a thousand. 
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. 
In prosperity he will make himself your equal, and be bold with your servants; but if you are brought low he will turn against you, and will hide himself from your presence. 
Keep yourself far from your enemies, and be on guard toward your friends. 
A faithful friend is a sturdy shelter: he that has found one has found a treasure. 
There is nothing so precious as a faithful friend, and no scales can measure his excellence. 
A faithful friend is an elixir of life; and those who fear the Lord will find him. 
Whoever fears the Lord directs his friendship aright, for as he is, so is his neighbor also. 

Trust, but verify – Russian Proverb.

My grandmother and mother always say 3 things about friendship –

“Show me your friends, and I will tell you who you are.”

“A friend in need is a friend indeed.”

“Betrayal is never by an enemy.”a

How many close friends have become strangers as you have grown? How many close friends have become enemies as you change? How many old friends are you more careful with, as you get more access?
It will not change as you progress in life. The cycle will continue.

To improve the quality of your friends, you must first improve yourself. You are the sieve, and what you keep, stays.

Friendship is one of the few golden experiences of life. If you test and find a true friend in life, grace abounds.