The OUP word of the year announcement has nothing to do with whether the word is new or not; it's based on its "currency and potential longevity". But simply because I can, I looked into how old it is. And like so many other words the kids today are using to destroy our language, there's nothing new about unfriend, at least in the past participle:
Will you with those infirmities she owes,
Vnfriended, new adopted to our hate,
Dow'rd with our curse, and stranger'd with our oath,
Take her or, leaue her.
- Shakespeare, King Lear I i
I hope, Sir, that we are not mutually Unfriended by this Difference which hath happened betwixt us.
- Thomas Fuller, The appeal of injured innocence III. xxxjb, 1659
All quotes I could find are of the past participle. It's possible that unfriended was derived from friended, the past participle of the verb friend (which dates from 1225). In that case, unfriend as a verb, as in "I decided to unfriend my roommate on Facebook", really is new.
As a noun it's even older:
We sollen wende and wid ham fihten, slean houre onfrendes
(We should go and fight with them, slay our unfriends [enemies])
Layamon's Brut, c1275