Learn English – A suitable equivalent for the phrase ‘give glad tidings’

I think this saying puts emphasis on unfairness and inequality.

So "it's an unfair world" would be an equivalent.

Also, Oxford dictionary has this translation.

http://www.oxforddictionaries.com/translate/spanish-english/dios

Additionaly, some sources mention the below proverb as an English proverb (which probably passed from Spanish or French in early 20th century). Not common though.

The gods send nuts to those who have no teeth.

From "Oxford Dictionary of Proverbs":

Said of opportunities or pleasures which come too late to be enjoyed.
Cf. Fr. le pain vient à qui les dents faillent, bread comes to those who lack teeth.

• God gives us nuts to crack when we no longer have teeth.
  _{[1929 American Speech IV. 463]}

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• The gods send nuts to those who have no teeth. In this life we either have too little of what we do want, or too much of what we don't want or can't use.
  _{[1967 Ridout & Witting English Proverbs Explained 68]}

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• ‘It's seeing the gardens I—we—would have been going for,’ insisted Sloan, ‘Not the luxury.’ ‘Quite right,’ said Leeyes, adding obscurely, ‘The nuts come when the teeth have gone.’
  _{[2000 ‘C. Aird’ Little Knell (2001) xiv. 161]}

Related to: old age; opportunity, missed

The term long-standing means

Having existed or continued for a long time: a long-standing tradition

Oxford Dictionaries Online

You could say most long-standing customer.

Perhaps you might also say longest-standing customer, but long-standing is a fairly fixed term, and that neologistic phrase might conjure up images of someone waiting in line for the most extended period.

P.S. I wouldn't use this term if you are in the seating business.