As others have mentioned, you've got options. I'll share pictures (as thumbnails, click to zoom) of each to make this maybe a little bit easier. On the adult menu, there's the Satu'li Bowls:
The one in the front is one of the combinations Cosmic Commando mentioned - grilled chicken over red and sweet potato hash topped with a sauce of choice and a vegetable slaw with yogurt boba balls. I agree that the grilled chicken and sliced beef over potato hash is probably a normal choice that would offer a lot of food. Disney Food Blog loves the beef bowl with potatoes. If they're picky, getting this item may mean asking for no slaw, and I think the sauce options will either excite (the onion sauce is a pretty standard deal) or scare away (black bean vinaigrette). If potatoes aren't their bag, there's a rice/quinoa option that's pretty easy too.
Also on the adult menu is the cheeseburger pod which a couple folks mentioned:
It's pretty easy. Ground beef, mustard, pickle, and cheese stuffed inside a "bao" or "steamed bun," the same kind of Asian pastry used in pork buns. These are reportedly a little bready but pretty good. It comes with two so it's no small amount of food and is cheaper than the bowls. On the downside, the sides here are vegetable chips and that same slaw. Although the veggie chips are effectively just "potato chips, but not just potatoes", the mere mention of them might scare off your picky eater. So, given the presence of both veggie chips and veggie slaw, this may turn out less for them to eat than a bowl with potato hash or rice would be.
Off the kid's menu, as others have shared, there's the "Teylu" wrapped hot-dog:
Which is basically a bagel dog. It's a parker roll, so it's apparently nice and fluffy on the inside, crunchy on the outside - and a little big more robust than your standard bagel or, say, croissant. The sides here are again sort of a let down for truly picky eaters - fruit and veggie chips - though these are so low in price compared to the other items that someone particularly hungry could grab two of these and forego any side dishes.
The same deal, regarding sides and price, applies to the quesadilla. Which, it's a quesadilla, you don't need a picture of that I think. It's just cheese in tortilla, nothing could go wrong. Based on preliminary reports, I'd have the picky eater avoid the love-it-or-hate-it Pandoran Sunrise drink, which is a mix of melon Powerade and other flavors. Refreshments won't be a problem otherwise - it's unclear how they've managed to do it, but Alpha Centuri Expeditions have brought Coke products and brand-name beers all the way from Earth to the restaurants in the Valley of Mo'ara.
Remember that Disney chefs can be quite accommodating with regard to exclusions and food allergies, especially if you call in advance. Also remember that you can use mobile ordering at Satu'li - for a picky eater, ordering ahead of time outside of the pressure of the restaurant and the line may make processing the menu a little easier.
I hope this goes well for your whole party! Have fun on Pandora!!