D&D 5E (2014) Hiding mid-battle; surprise or not surprise?

[guachi]

I occasionally resolve a surprise attack on PCs before rolling initiative so the players are actually surprised and react more as if they are surprised. If any of them wish to use a reaction on round one we roll initiative at that time. It adds a bit of fun (at least it seems to be more fun).

"An arrow flies towards you!"
"I cast shield"
"Roll initiative."
<Dice rolls>
"Oh, no. the arrow hit me before I could get off my shield spell!"

It is, technically, wrong. But it does add some dramatic tension. And that's not a bad thing.

However, getting back to the OP's question: Like everyone else has said, your player is wrong. Once combat starts there is no more chance for surprise.

 

[discosoc]

I have allowed a chance at a player gaining surprise if they are entering an existing combat from surprise for the first time (ie. not participated in any previous rounds and Stealth beats all Passive Perceptions). I have had some assassins try to convince me that they automatically get their surprise attack if they beat the opponents Initiative that first round, but really they only get the Advantage on that attack.

5e treats "surprise" more like a condition that everyone starts with on the first round of combat than an effect that you gain by fulfilling certain conditions. It's basically impossible to surprise someone who's already involved in combat -- in the mechanical sense of the word -- regardless of whether or not they know you exist. The benefit of being attacked by someone you didn't know was there isn't handled by surprise; it's handled by the unseen attacker rule (advantage to attack).

I think surprise is something that 5e really messed up on, even though it makes total sense to me. There's just too many people that can't seem to wrap their heads around the fact that there's no surprise round anymore, and surprise isn't something you gain on someone else, leading to weird rulings like in the quote -- rulings that make perfect sense as long as you don't understand the mechanics.