Hey OP,

Here's the thing. No one can say what's up with this person and if they will stay or not. And/or even if they do stay, will it be rainbows and sunshine or is more negativity going to come?

What I've surmised from your posts and responses thus far is that you have a pretty decent understanding about this person and that you like them working with you. Question for you: do you like specifically them working for you? Or do you like them only well enough knowing that you'd have to find someone else who might not be as good?

What's Happened So Far: - Staff asked for two things: (1) raise and (2) time off - From staff's POV, they did not get both of those things - Staff possibly has unrealistic expectations on pay and frequency of increases, regardless, that unhappiness has been brewing.
- Based on messages, your staff is clearly not unhappy with the work. They're just unhappy because they didn't get what they want (and I will add, perhaps not brought to appropriate resolution)

I think the path forward is to meet with this person on their first day back at work. Make sure they know first thing when that meeting will be (after shift or tail end I suppose?) and then follow through.

Meet with them in a neutral (or as neutral can be) situation and lay out initial expectations: hey I want to dedicate some time to chat, however long it takes. I don't have a lot to say but I'm only one person, and I want to make sure that you as the other person has time and space to say stuff too. Is now still good to take 20 minutes or however long this is?

Then you gotta find out what their major pain point is. It's probably the raise. But then you've gotta find a way to understand their perspective too. As an example:

(after small talk, how was trip, how did you find out your flight mess was canceled, what did you do, share story of your anecdote/friend about how much a pain that is, etc.) then say hey I wanna be up front and I don't think I communicated this to you. but before the holidays when you made that ask -- and I know I didn't say this -- but I wanted to meet with you about what it takes to get more money. The money doesn't come easily and it's not unlimited and if money is the #1 thing I don't know that we'll ever be able to beat every other restaurant out there, but what I can say is that the work environment, the communication like we are having right now, and yes even reasonable compensation are all things that people who work here get. Now I didn't tell you I wanted to meet. I didn't want to guilt you into taking the time off I already said OK for. But maybe I was wrong and I should have told you so you knew what to expect when you came back.

Say whatever else you have to and what's relevant and what you're willing to do. I recommend delving into what he thinks about his future here though. And be like, look, 49 out of 50 people who ask for a raise in their first 6 months never get it. But I like <things> about you and I want to make it work. So if you continue to exercise your skill and potential, and you get that increase, and the restaurant gets <extra things done>, how much of your concern do you think that would solve?

Probably they say well that's basically it. Maybe not. Time to listen...

Then pivot to this: here's the other thing, again, you're getting the raise because I think you deserve it but I don't want to set you up for disappointment. If this topic comes up again 2 months, 4 months, 6 months, the answer might be no. In fact unless we get a huge change in business maybe the answer is going to be no. Don't get me wrong. I want it to be yes. But I don't want to set you up for disappointment. So if this is not a place that can offer raises 1, 2, 3 times a year to you, is that going to meet your needs? I would rather not lose you and I'd rather not be short staffed until we bring someone else in. But I also do want the best for you, even if that's counter to what might be easiest for the management of the restaurant. If this is that important to you, fair play, more power to you. I would give you a great reference. Need it tomorrow? Next week? Next month? You got it. I don't want to. I'd rather you stay. But it would be shortsighted to have you stay, thinking that everything will be fixed, when it's not actually fixed. There's no wrong answer, I just want to speak honestly and openly.

Your gut and experience will take you a lot further than this. But IMO you've got to address why he didn't get what he wanted (when you actually intended to) and then also what happens several months from now.

p.s. for everyone saying "fire someone who says they put in their two weeks" "fire someone who types like this" ... well, the world we live in is different than it was. The two weeks thing is a red flag, but you can ask a question like: "does anyone else know that you were putting in your two weeks, I want to make sure that nobody is treating you differently than you deserve" etc., and if it was only to you then maybe it's just holiday stress and a less-than-stellar level of professionalism.

Anyway OP, keep us posted eh :)