El que avisa no es traidor

El que avisa no es traidor
 

How do we say that in English?
El que avisa no es traidor.
He who warns is not a traitor.

Your translation is fine, but the literal translation doesn't appear to be a popular saying in English.

Two translations that are offered online are
"He who warns is not scorned" and
"He who warns is excused." (This last one is a translation of an Arabic phrase.)

If you'd rather use a popular saying, try
"Don't say I didn't warn you,"
"Don't say you weren't warned,"
"Don't say I haven't warned you," or
"Don't say you haven't been warned."

Even the simple "You have been warned," is a good translation.

"Forewarned is forearmed," a translation of
"Praemonitus praemunitus,"
could also be used, but isn't as popular a saying as those mentioned above.

Rusty :
He who is warned is not betrayed, is this sentence correct?
Thanks

It is an appropriate translation, to be sure, but is not a popular English saying.

What's the meaning of this idiom? Can you tell? The translation seems fine, but as someone answered above, you may find a better way to say it in english. It depends also where and who you're speaking to (american, british?)

The OP is from Spain, so he is in general contact with British English.
Some, if not all, of the translations I provided work in either hemisphere.

Asking what an expression means is a good course of action, since no meaning was provided.

I first searched for the meaning of the expression before providing the translations I gave.

Idioms, sayings and expressions in one language often have equivalents in another. The trick is learning how they are expressed in the other tongue.
A word-for-word or a literal translation seldom works.

I assume it means don't shoot the messenger.

No, the Spanish phrase doesn't mean that.

Well, if you treat the guy who presents the bad news as a traitor, it seems similar to shooting the messenger. It is true that there is a difference between someone who presents the bad news and someone who warns of bad events, but I think in many cases they are interchangeable.

"Shooting the messenger" has nothing to do with "el que avisa no es traidor". It seems to be some misunderstanding there about what "avisar" means: it isn't to convey a message or bad news -then maybe becoming the scapegoat accountable for those bad news- but to warn or notify about the future actions of the very same person issuing the warning.

The Spanish idiom says you shouldn't resent the actions of someone who beforehand has warned you about doing so.

"I have your best interests at heart"?
"Friends don't let friends...(do something foolhardy)"?
"I'm telling you for your own good"?

Thank you, Rusty.

"El que avisa no es traidor" implies a confrontational set-up ... at least, potentially. Not necessarily antagonizing, but certainly not friendly.

Certainly no English expression like "don't shoot the messenger", "forewarn is forearmed" or "I'm telling you for your own good" are a match or are even close.

In that case I was wrong to propose the suggestions I did, having misunderstood the direction of the traición. Sorry!
So how about "You've been given fair warning"?

I think that translation is good and it's similar to several found in post #2.