Forum:Compulsory Evacuation Device vs Flip effect

Hi, this is my first time posting here, so sorry for anything I've done incorrectly. Anyways, to the point. My friend and I were just dueling and we came across a little argument. The story goes I have a face down flip effect monster on the field (it was an Old Vindictive Magician) and my friend has a face down monster and trap card. Now, as I am flipping my card (it is literally half way flipped) my friend decides to activate his face down trap, Compulsory Evacuation Device. Here's where the argument starts. He insists that since my card wasn't flipped all the way, he could activate Compulsory Evacuation Device before I had it fully flipped and that it would go back to my hand, effect unresolved. Now personally, I think that's unfair and should be against the rules, waiting to see if they're going to flip it first. Now I really have no idea (which is why I'm posting here) if it really is just whoever can activate it first, or if there's some kind of priority involved, but I would really like to have this solved. Thank you.