" Double negatives, when used to express a negative idea, aren’t acceptable in standard English and you should avoid them in all but very informal situations (or when singing along to pop songs). "

Quite so!

Here's a game: Finish the following lines from popular songs and name the artist:

I can't get no....

We don't need no...

I don't want nobody else..

Actually, double negatives do exist in English.

Here is a quote from the Oxford Dictionary blog:

After the 17th century, certain writers attempted to make English spelling and grammar more systematic, and relate the rules of language to those of logic. The Oxford English Dictionary records that in 1775, Lowth’s A Short Introduction to English Grammar stated:

Two Negatives in English destroy one another, or are equivalent to an Affirmative.

This edict had an impressive staying power and remains the case today. Double negatives, when used to express a negative idea, aren’t acceptable in standard English and you should avoid them in all but very informal situations (or when singing along to pop songs).

Millions of native speakers do use double negatives to express negative ideas. They are very common in the speech of working class people in the UK. And in the US too. There is a difference between using them as a native speaker and using them as a non-native speaker though. 

A native speaker says 'I don't need you no more' and it's just part of their way of speaking. People do get judged, rightly or wrongly, for speaking that way though. However, if a non-native speaker does it, people will just think your English isn't great. Not that it's dialectal. Harsh, but that's how it is when you don't have a native accent.

The original sentence can't have been written by a native speaker, the grammar is all over the place.

A double negative actually becomes a positive, so this sentence is saying that we must help  and lend money to John, or literally, we must not not help and not lend money to John. 

My advice is to not use double negatives, but don't not get used to them because many English speakers are unaware they are using them, so it's good to understand they really accidentally add an extra negative. 

e.g. I ain't got no money. (Terrible grammar, but we all know what it means, so you should too)

There are also times when double negatives can be a bit creative, or an evasive method of not really answering a question. 

e.g. "Do you love me? "

"Well, I don't not love you." :)