English Translation
He never told anyone.
I was wondering why “se” is used here. I think the reason is:
“se” in this context takes on the meaning of “les” (to them).
In a nutshell, when “les” and “lo” are combined, “les” changes to “se” as is the case in this sentence. See this topic for a more detailed explanation.
I’m wondering how “se” (from “les”?) and “a nadie” can work in the same sentence?
I currently understand the sentence as
“He never said it to them (to) nobody.”
That obviously seems odd. How can the sentence simultaneously contain “to them” and “nobody”?
Thanks in advance. 
See also: Le dije a la policía todo lo que sé.