After living this experience for nearly fifty years, this is indeed wise advice. True friends are very difficult to find.
When I was in a low place in my life, I had a friend tell me a story. I'm sure this is probably somewhere on the Net, but...
"So you're walking along on the road of life, and you're not paying attention to where you're going. You end up falling in very deep pit. You attempt to climb out, but the walls of the pit are rather smooth, and there's nothing that you can use to climb on. So, you call out 'help! help!'.
Eventually, somebody comes along and shouts down to you, and you reply, 'hey, I need some help.' The man pulls his wallet out of his pocket, fishes around inside it, and produces a card, to which he tosses it down to you, and then walks on his merry way. He gave you a business card to his law firm.
Angry that this person didn't help you, you begin shouting again, 'help!, help!!'. It's in the middle of the afternoon now, and you're getting thirsty from all the shouting you're doing. Finally, another person peers down the hole down at you. 'Hey, can you please help me out of here?', you ask. She reaches in her pocket and pulls out a pad of paper, scribbles something on it, tears the page off and tosses it down to you, then proceeds to walk away. She was a doctor, and wrote you a prescription for the pain you must surely be in.
Furious that she didn't offer you any real help either, you begin cursing and screaming obscenities, yelling at the top of your lungs for help. You're voice is becoming hoarse now from all this shouting, and it's getting late. The sun is beginning to set. You begin to cry. But somebody peers down to you and says, 'hey, what are you doing down there?' And before you have a chance to answer, this person jumps down into the pit with you. You go from tears to pure anger at the stupidity of what you just witnessed. 'What did you do that for?! Now we're both stuck down here?!' This person looks at you and grins, as they stand up and offer a hand to help you stand up. 'Yeah, I know. But I fell down here last week. And I know the way out. Come on, I'll show you.'"
He told me this and I honestly wept. Years later, as I write this now, I feel tears coming to my eyes, and after much more reflection on this story, I've observed that the other two people were trying to help, in the ways they knew how. And they may have been pretty smart, but never fallen down into a hole (or a dark place in their life). While that last person, you're friend, will be there for you at two in the morning to help you get out of the mud, or bail you out of jail, or to bring you some left overs for dinner, has experienced some of life's little pitfalls, knows best how to truly help. These people are rare, and what's more rare is the fact that most people would rather hide the times they fell into these pits, and by helping somebody, as the third person did, exposes these "pitiful" parts in their lives.
Anyway, once you find somebody who is open and honest like this last person, who would put themselves in harms way to help you, to teach you, to ensure you're okay, never let them out of your life, and be sure to be there for them too!
(Wow, that was long-winded.)