One word for a person who pretends to be nice to someone's face but makes fun of them behind their back [duplicate]

Solution 1:

Echoing Jim's comment above, you might consider 'two-faced'

• Hypocritical or double-dealing; deceitful.

• Being or acting so as to conceal one's real intentions

_{^{http://www.thefreedictionary.com/two-faced}}

Specifically this would fit with someone who quite literally shows one face (friendly) and another face (unfriendly) under differing circumstances.

So the usage would be

"I hate how two-faced she is, always talking about people behind their backs"

Solution 2:

If you are looking for a single word, then I would suggest backstabber

Meaning

Noun. (plural backstabbers) A traitor or hypocrite, such as a co-worker or friend assumed trustworthy but who figuratively attacks when one's back is turned.

(YourDictionary.com)

Solution 3:

There is also

From WordNet (r) 3.1 (2006):

duplicitous, adj

1: marked by deliberate deceptiveness especially by pretending one set of feelings and acting under the influence of another;
"she was a deceitful scheming little thing" - Israel Zangwill;
"a double-dealing double agent";
"a double-faced infernal traitor and schemer" - W.M.Thackeray

[syn: {ambidextrous}, {deceitful}, {double-dealing}, {duplicitous}, {Janus-faced}, {two-faced}, {double-faced}, {double-tongued}]