Used as an adjective, backup is one word (sometimes hyphenated, especially in older works), not two.
“I’m just the backup friend.”
Backup is never a verb. You can back up your computer’s files, or back them up, but you cannot backup them.
I see a lot (never alot — please stop that!) of confusion (or indifference — it’s hard to tell) about when to use the adjective form (verb and preposition as one word) and when to use the verb form (separate words). Here are a few examples: You can work out, but you cannot workout even if you have a daily workout routine. You can sign up for an email newsletter from your favorite blogger, but you cannot signup for it even if you fill out the sign-up form. You can make up your face for a Halloween costume, but you cannot makeup your face, no matter how much makeup you apply.
Now… if you really, really want to use the exact words from the original example, and you’re willing to make changes to the punctuation and thus to the meaning of those words, there is a solution: “I’m just the — back up, friend.” Here, we have no idea what the speaker was about to say he is, but we can infer that someone got too close, and the speaker had to interrupt himself to tell that person to back up.I’m fairly sure, though, that the writer of the original intended the speaker to say that he’s just the friend to call on if the first-choice friend is unavailable. Or, I suppose, you could add a comma to the other version and have him say, “I’m just the backup, friend.” Now, we know he’s the backup, although not for what, and he’s still addressing the person to whom he’s speaking as friend.
See how much difference punctuation can make even in a single, short sentence?