expression, godan verb (-ru), intransitive verb
A rare, euphemistic expression used to avoid saying 死ぬ (to die). It literally means 'to become auspicious', but is used ironically or superstitiously to refer to death.
彼はとうとうおめでたくなったそうだ。
I heard he finally passed away.
The standard, direct word for 'to die'. おめでたくなる is a rare, indirect euphemism.
A common polite euphemism for 'to die'. おめでたくなる is much rarer and more colloquial/ironic.
From おめでたい (auspicious, happy) + なる (to become). The ironic use as a euphemism for death likely arises from superstitious avoidance of directly mentioning death.