Even though it is a french word, it is used almost exclusively in English sentences. It is also used according to English grammar therefore one would say "I avou-ed"
It is used in certain social circles
1) to gain approval on what one just said from your peers to which they'd usually reply "I avou (it)"
2) to demonstrate your approval regarding what your friend just said.
-I gotta avou that's brutal.
-Avou you'd bang her any day.
-I avou. (it)
-She told me i was cheating and I totally avou-ed that.