I have a friend who enjoys Historical and Historical Fantasy TV series, but she's never watched a Korean one before.  And I'm having a hard time choosing what to send her.  

Which one of these would you recommend for a first-timer?

Faith
Gu Family Book
Iljimae
Scholar Who Walks the Night
Sungkyunkwan Scandal
Time Slip Dr. Jin

Don't know exactly what your friend's preferences are...like medical, or romance, or action, but I enjoyed them in this order: Sungkyunkwan Scandal, Iljimae, Gu Familly Book, Scholar, Faith, Time Slip
in my opinion you know better what your friend's preferences are but cause she has never watched a k -drama before i think gu family book would be ok. 
Sungkyunkwan Scandal, maybe. But I'd recommend Queen In Hyun's Man or Rooftop Prince first so she can get used to watching korean historical settings/terms. They both are timetravel dramas taking place in present and past.
I recommend Rooftop Prince and Faith first. Rooftop Prince because this drama is a complete package, drama, comedy, fantasy, action and romance, also bromance. Lol next is Faith, if your friend knows BOF and liked it, or he/she likes LMH, this a great drama as well. P.S. i hate historical dramas but this time travel dramas made me love historical dramas because somehow, the fantasy add some spice to Korean History and made it more fun and interesting. ^^