In that context, as far as I can tell, it should be "A friend of mine". But what is a "down slide"? "A friend of mine's + noun" may be tolerable.
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Little Girlsome troll managed to hack into the forum and deleted some posts.We regret the loss of a few threads due to a malevolent forum member. The technical vulnerability has been patched.
Little GirlI always thought it is "a friend of mine" until recently I saw "...of mine's" on BBC, and have been somewhat confused since then. Shou