I know the writer meant heart-rending, and perhaps it was a simple typo; sometimes when we're typing fast our fingers type a word the way they want it typed when our mind is thinking something else. But perhaps this person actually thinks "rendering" was the correct word to use.
I offer a quick lesson on rending and rendering.
Render can mean several things. The root render means to give or hand over. As a transitive verb it can mean to submit or present (I rendered the bill to the customer.), to give (I rendered assistance to the stalled driver.), to give something owed (Let us render thanks to God.), to give in retribution (I rendered an apology for my rudeness.), to yield (I rendered the field to my opponent.), to represent, either in verbal form or in a drawing (I have tried to render my own feelings on the opera; My drawing did not render the subject well.), in computer science, to convert from a file into a visual form (I rendered the graphics file into a video display.), to perform or arrange in music (My own rendering of the musical piece was different from the composer's; I rendered the composition for a string ensemble.), to translate (I was able to render a translation that was accurate.), to pronounce (The jury rendered its verdict), to cause to become (Your news rendered me speechless!), and to reduce or melt down fat by heating (My grandfather used to render hog fat into lard.), and as a nautical term, to slacken (I rendered the rope for the captain of the ship.).
Rendering can be a noun, meaning the payment given for a good or service. There is also renderable, an adjective, and renderer, a noun.
None of those definitions would have been correct in the Facebook example I cite at the beginning of this post.
To rend something is to tear it or break it apart; it is to wrest, to divide, to pierce with sound, and to cause pain or distress.
Now heart-rending, on the other hand, is an adjective that means "causing or marked by grief or anguish". It is a synonym for heartbreaking and sorrowful. (And apparently it can be written as one word or as a hyphenated word.) I'm sure that is the expression the writer on Facebook was rendering to her friend.
But it was heartrending to me to see rendering used incorrectly.