popped off t and n and now he broke the retainer

@873655, What exactly did your brother break? Generally if it's the keyboard hinge you can buy individual replacements that can include the key, the hinge and the cup. So unless it's the part on the keyboard frame itself that's broken, you can replace it. Here's one example of where you can get the part, assuming we're still talking about an Asus C423NA Chromebook.

Asus ChromeBook C423NA Keyboard Key Replacement - ReplacementLaptopKeys.com

Note that this particular model comes with two different kinds of keyboards, so you have to be sure to order the correct one, Version A or Version B.

If that's not your Chromebook model, search around on that site; they carry a LOT of different keys.