Single word request: Harming the benefactor

Such a person would be called a backstabber:

Backstabbing: the action or practice of criticizing someone in a treacherous manner while feigning friendship.

I chose this definition because unlike some others I found, it includes the element of false friendship, which I think is essential. One cannot backstab an enemy.

Not a single word, but an idiom that is so closely related I had to suggest it:

Bite the hand that feeds you

Your friend has bitten the hand that feeds him.

From The American Heritage Dictionary of Idioms

Show ingratitude, turn against a benefactor. For example, The college gave me a scholarship, so I shouldn't bite the hand that feeds me and criticize its hiring policies . Used about 600 b.c. by the Greek poet Sappho, this metaphor of a dog biting its master was first recorded in English in 1711.

The very fact that the definitions of this idiom do not give a single word definition suggests that there might not be a commonly used single word (other answers may prove me wrong, though!).

Most would call them a "traitor" or describe them (or the deed) using a related word like "treachery or treacherous". These describe deep betrayal of trust and good will, plain and simple. See also "Judas".

https://www.thefreedictionary.com/traitor

https://www.thefreedictionary.com/treachery

https://www.thefreedictionary.com/judas

"Ungrateful" is also suggested as a translation of কৃতঘ্ন kr̥taghna. As a noun, one might call such person an ingrate or that they are exhibiting ingratitude or ungratefulness.