What is a word that means "someone who pretends to be your friend but is actually your enemy?"

There are lots of possible answers here.

This is quite a common social dynamic, so a lot of words have been discovered to be relevant.

Frenemy is a portmanteau that exactly describes this situation.

frenemy noun - one who pretends to be a friend but is actually an enemy [Merriam-Webster]

Personally though, I think this a rather cringey word, I'd much sooner use some of the other suggestions.

From your description, your friend is two-faced (duplicitous). He may also be more generally hypocritical.

In the US, he might be called a Benedict Arnold.

The word mouth-friend is one possible answer.

After you discover how he has turned on you, you might call him:

• a false friend [not to be confused with the linguistic term of the same name]:
  not faithful or loyal : treacherous <a false friend> (m-w.com "false", def. 4a)
• a traitor:
  one who betrays another's trust or is false to an obligation or duty (m-w.com def 1)
• a backstabber:
  [one who engages in] betrayal (as by a verbal attack against one not present) especially by a false friend (m-w.com def. 1)
• a double-crosser:
  to cheat or deceive (someone) especially by doing something that is different from what you said you would do (m-w.com def. 1)
• a two-timer:
  1: a double-crosser
  2: one who betrays (a spouse or lover) by secret lovemaking with another (m-w.com)

• disloyal:
  failing to support or be true to someone or something : not loyal or faithful to your friends, family, country, etc. (m-w.com)

• perfidious:
  not able to be trusted : showing that someone cannot be trusted : characterized by perfidy (the act of betraying someone or something : the state of not being loyal) (m-w.com)

The general answer is traitor or backstabber, but other answers are also possible, depending on factors such as the order of when they become your friend or enemy and the severity of the treachery.

If they were an enemy from the very beginning but approached you as a friend, working for someone else all along, they’d be a spy.

If they did not see you as an enemy but merely saw you as an asset, they’d be a two-timer.

But if all else fails, you can always resort to the good old d’bag or a’hole.

There’s also a wolf in sheep’s clothing, which is a person who seems friendly but in truth is hostile.

Machiavellian

While Niccolò Machiavelli has got some bad press, his name is used to describe a person who is

• Cunning, scheming, and unscrupulous ... Oxford

• ... characterized by expediency, deceit, and cunning. Yahoo

• ... cunning, amoral, and opportunist ... Collins

Such a person is nobody's friend. At best they treat you as a means to an end, or an ally of convenience.

While not necessarily an enemy or even a bad person, a sociopath can fit the bill when it comes to manipulation, lying, and lacking moral fiber.

If betrayal is the most important point to get across, then turncoat comes to mind, though it doesn't imply premeditation. Viper is often used to describe someone who hides their intentions and lies in wait until the ideal moment to strike.

Check out the answers to this question: Metaphors similar to "Trojan Horse", many of them seem to fit your requirements.

If the cultural reference is appropriate to you and your audience, he's a Judas (after Judas Isciarot, who betrayed Jesus to the Romans).

Here are some adjectives you may find useful:

• Deceitful; guilty of or involving deceit; deceiving or misleading others.

• Mendacious; not telling the truth; lying.

• Perfidious; deceitful and untrustworthy.

• Sly; having or showing a cunning and deceitful nature.

• Surreptitious; kept secret, especially because it would not be approved of.

• Treacherous; guilty of or involving betrayal or deception.

• Duplicitous; deceitful.

• Underhand; acting or done in a secret or dishonest way.

• Unscrupulous; having or showing no moral principles; not honest or fair.

I understand some of these are synonyms, but you may find one you especially like.

Con artist or Confidence man / Link #2

Term is used in social engineering for person who excel in psychological manipulation. In general, modus operandi is - pretending to be friendly while having malicious goal(s)* Probably, the the most well known con artits is Kevin Mitnick

*just to be clear, it doesn't mean con artist is a "bad" person by default, e.g., security consultants

For a spy story, these are the words that came to my mind

enemy agent - typically you would use this for a known enemy.

secret agent - secret agents you just don't know about - who they work for is a secret.

double agent - this one is usually used when the agent is spying for both sides at the same time. It's most typically applied when someone has been "turned" away from the side they originally worked, and is now secretly working for the other side, while pretending everything is still normal and they are still working for you. This is a very common term in spy stories and will be easily understood by readers

turncoat - typically used for someone who visibly switches sides.

defector - typically used for someone who officially changes sides, declaring their intent in some official manner such as gaining citizenship in the enemy country.

mole - someone working inside your agency, pretending to work for you, but secretly working for the other side. Same as a double agent, but "mole" is used for more drama - there's an insidious factor about a mole, that doesn't exist with a double agent. This would be a good one - it's used a ton on American TV and I've seen it used in the British series "MI-6" as well. This one is so well known, there was a whole game show called "The Mole" where a group of people had to figure out who was secretly working against them.

Or, if you want to get Shakespearian, you could call him an Iago (although that might be bit obscure for some).

In the play Othello, Iago is Othello’s trusted advisor, who is plotting to see Othello destroyed.

Double-dealing - the practice of working to people's disadvantage behind their backs

Used as an adjective: "he is a backstabbing, double-dealing twister"

Brutus
Someone who is a Brutus is a very good friend who gives you the screw job.
This is derived from Julius Caesar's best friend. His best friend Brutus was the last person to stab the dictator Julius Caesar's last words were "Et Tu, Brute?" which means "You too Brutus?"

Source website

Sounds like you're thinking of a

Snake in the grass

One who feigns friendship with the intent to deceive.

from The Free Dictionary