Originally Posted by abaka
The plain English is "she is Jack's friend". I think that's best form of all for a monosyllable.
Still better, She's one of Jack's friends.
Originally Posted by Soup
As a native English speaker, I would never use a construction like 'a friend of Jack' nor have I ever heard a native English speaker use such a construction.
a friend of Jack's
a friend of Jack
Originally Posted by svartnik
Nope, not kidding. Why would I kid?