Friend A could reply, "I am....:

>[Flabbergasted](http://www.thefreedictionary.com/flabbergasted) — overcome with astonishment; amazed; astounded
>
>[Thunderstruck](http://www.thefreedictionary.com/thunderstruck) — affected with sudden astonishment or amazement
>
>[Gobsmacked](http://www.thefreedictionary.com/gobsmacked) — utterly astounded *(British slang)*
>
>[Floored](http://www.thefreedictionary.com/floored) — stunned; overwhelmed: *The very idea floored me.*

I think gobsmacked, even though I am in the U.S., is my favorite.