As someone said upthread, what happens to him depends on him. Upon discovery of the flask, Security will give your friend the option of returning the flask to the car (or resort, if you didn't drive and are on site) or they will have him empty it at the front gate. They're not likely to toss the flask unless your friend says, "Never mind -- just toss it." My suggestion would be to park as far in the back of the netherlands of the parking lot as possible, so that when he chooses to return the flask to the car it takes him a long looooooong time.
On the chance that the flask is not discovered during bag check, you may want to mention to your friend that there are undercover security CMs all over property. And should one of them see him swigging from his flask inside the park, it would not necessarily lead to him being kicked out (again ... that's up to him), but it will definitely impact his day since he'd at the very least get the "lecture" and be asked to dump the flask.
Honestly? I'd tell him that he'll be on his own on "flask day", and that if he wants to join the rest of you as you tour MK, then the flask stays at home.
It'd actually be pretty easy. An airline bottle of liquor would be pretty easy to pocket and hide. Last time I checked, Security didn't do pat-downs.
I didn't see anywhere that the OP said the friend couldn't manage to spend a day at the MK and not have a drink, only that the friend was going to sneak in a flask. Two
totally different things! Lots of people try to sneak liquor into the MK -- mostly because they know its a 'dry' park and they want to see if they can. It's more about the sneaking and the "doing something that is forbidden" thing than it is about being an alcoholic. I've only ever seen one guy actually succeed. He had brought in a bottle of Sprite that was heavily spiked with either gin or vodka. According to his friend, he'd been paranoid all day about getting caught, so instead of just carrying it around all day as planned (just to prove that you could bring alcohol into the park and not get caught), he drank it almost as soon as they got past bag check. First ride that day? Teacups. How do I know? I was working Custodial.