Recently, I was trying to rescue two people in an elevator.
The female Krogan and Mordin
This being Mass Effect, there were bad guys trying to stop me. One of them, in the course of trying to stop me, tossed a grenade into a wall, where it bounced off and landed at his feet (not his fault, he came down with a sudden case of shot-in-the-face-itis)
Unfortunately, so did I, and as a result I didn't get to see if he would be "hoist 'pon his own petard", as it were.
Do opponents in ME3 take damage from their own grenades?