It seems to me there's some conflicting priorities going around.
On the one hand, a desire for realism. Or verisimilitude or whatever it's called this week. People, in a surprise situation, ought not to be able to dodge as well as in normal situations. Then again there's something to be said for the Monk and the Barbarian being both at least quasi-supernatural, so even though the desire is understandable, in this particular instance its applicability is a little weak.
Then there's game balance. This is not an untested UA article we're talking about or a simple ruling about an open norm, it's a houserule that nerfs a class feature. Those features were designed, put in, and worded as they are for a reason. Simply brushing their design aside as a "mistake" (as opposed to simply not being to your liking) is quite a claim.
There's also security to consider. For all the insistence that a player ought to be "flexible", it's difficult to maintain enthusiasm for a game where class abilities get nerfed with houserules after the game has already begun. Especially when the justification isn't game balance or official errata, but simply the DM's whims for realism... when that realism is somewhat questionable, as mentioned earlier. (And as an aside, I bet that Wizards never ever have to deal with a nerf like this at the OP's table. Wizard-magic is immune to common sense, whereas Monk-magic somehow isn't.)
Canucksaram, I don't know you or your players. But given that someone is going to dislike this issue, no matter the outcome, why not make your case to the player and let them decide? Why do they have to be flexible to accommodate you, when it could also be the other way around?