If your boarfiend vanishes without updating, return to Madam Victra for a new boarfiend. #**Possible bug: The quest will not update even after gettina near boardfiend. If this occurs, delete the quest and pick it up again.
The quest will also only update if you are in the correct area (as shown by the RoI on the map).
If you are having trouble getting the update, try killing the fungusmen one at a time. Killing them two or three at a time or being in combat when the boar explodes seems to mess up the quest, and you'll have to restart.
7/8/2012: After having tried everything I've read online about how to get this update, I gave up on these tricks and received the update while killing swiftly (with a merc), while in combat, and after having gotten a new boar multiple times without deleting and restarting the quest. Just get a new boar and keep trying until you get the update.
5/21/2015: The quest works! You MUST KILL 10 mushroom guys to get the update. Ensure you are in the correct area and single pull them to be sure.
4/1/2018: Deleted the quest 3 times. On fourth attempt it worked. I only needed to kill 3 mushrooms when I got the update and I killed 2 of them with AoE. I got the update when I was stood at the POI loc of 350, -46, 260. (I had also just dinged lvl 15).