Plural or singular? Lesson 13.1

In lesson 13.1, the last line of the dialogue has Anne saying:

"Allez, sois courageux !
Sinon, tu vas rester vieux garçon ! "

Since they are friends they are using the "tu" form throughout, so why "Allez"?  Is that just a colloquial use?
 

"Allez" is the imperative tense ( command tense ) of the verb to go. 

True, but it's the plural imperative. The singular (familiar) imperative is "va!".

Since she uses "sois courageux" (singular, familar) rather than "soyez courageux", I just thought it was inconsistent.

Allez ! is also used as an interjection. 
Exprime l'encouragement, l'affection, la menace, etc. : Allez ! on se retrouvera.
It would be translated as come on! ("Come on, get up")

Ok thanks.