Well, he could've become deaf/mute, or his friend blind later during their friendship, so I hardly think the arab friend will take that response as valid, however I see your point here.
Uhm. One can't speak, nor hear, the other cannot see.
There is no link, no middleman. So sound and sight are impossible.
Using common sense I hardly think the stink of rotten meat would do the trick either. Leaving us to taste and senses. Taste is out of question..
I'm back at poking morse code now..
EDIT: Unless they're supernatural, in which case they'll just either use telepathic powers, OR are healed by Jesus.
EDIT 2: I know this has been suggested once, but I assume killing the friend would be a suggestion as well. That way he'd meet his mother in heaven (given the riddle was originally asked by a religious person, or someone who believes in the afterlife).