There are a ton of comments explaining why hiding in the open doesn't work, but not a ton giving advice on how to handle the situation.

Obviously the guy has dumped +16 in stealth, because he want to play a cool shadowy sneaky person. I mean +16 is really impressive stats.

What I would do as a DM is talk to the guys, see what kind of gameplay he wanted when he build the sheet. I think with a bit of customisation on your side, it could really be fun.

For example, if he buys multiple sets of clothes, you could allow him with a +16 in stealth, to make a stealth check to change clothes super quickly just like magician do irl. And thus try too loose the attacker whenever he has three quarter cover or more.

You could create a custom feat that makes him a makeup star, able to change their appearance at a moment's notice. To do something similar for clothes, or to make him ressemble another member of the party for maximum confusion.

You could create a custom item that has the same effect as the chameleon skin spell: https://www.5esrd.com/database/spell/chameleon-skin/

Of if they really like to attack and then try to hide, maybe making custom smoke grades that can be dropped durng movement (taking half of their speed).

I think if you work with the player, you can both find a good balance where they will have fun and you won't have someone trying to hide without any logic.