EzekielRaiden
Follower of the Way
Really wish more folks would consider the option of "this person IS the DM, and is looking for analysis or advice." Because I swear, this response comes up in the first 5-10 posts of every single thread ever about any rules question, and at least half the time, it's a DM wanting other DMs' thoughts to help guide their choice, making this response less than useless.What does your GM say, they have the final say no matter what is debated here? It is based on the Initiative.
Now, you have every situation to think about.
As for my own perspective:
If the player who readied the magic missile (or some other instant-cast spell) set an appropriate trigger, I'd allow it. E.g., "I ready magic missile for when the other character casts a spell" or "attempts to turn invisibile" or the like. However, you can't have it "both ways" as it were, in terms of affecting the target--you don't get to say that the spell hasn't taken effect yet so you're allowed to hit it, but has taken effect so you can force a Concentration save. One or the other: either the spell isn't up yet, so you're able to target the creature (but the creature isn't Concentrating yet), or the spell is up and you're having to deal with the "you don't know where it is" problem.