Some time ago we were in a final battle with a end boss for a dungeon. The fight went pretty well in our favor but when the boss hit his bloodied value, our DM described that he emitted a dark shroud into the room. Unknown to us what the shroud was we decided to continue our efforts to kill the boss. When one of my party members got low on hp I tried to heal him as the cleric of the party. Because of my paragorn path (Compassionate Healer) I was able to heal him for 12 + healing surge + 7d6 with a healing word, which turned out to be 57 hp. Because the dark shroud seemed to reverse healing so that it instead damaged the PC of my friend had to start making death savings trows. He failed the death saving throws and died because I tried to heal him.
The DM told us we could have known what the shroud was if we would have tried checking the shroud with an Arcana check.
I actually thought it was kind of funny but my friend thinks otherwise and now refuses to play with campaigns hosted by our DM.
My question: Is this a legitimate mechanic for the DM to use? If not, could it be used in another fashion or is it just outright wrong?
Details: DND4E, Party around level 13, DM makes every monster and encounter himself, first DND campaign experience for everyone in the party including the DM