The ammonia started to accumulate, unfortunately, as soon as he was put in because he will urinate and poo and there isn't enough beneficial bacteria yet to consume the ammonia. If its been a week, there's probably a good amount of ammonia in there now. He's probably somewhat uncomfortable cuz ammonia burns. There are old school types who think that cycling with a fish in there is fine, and they use "hardy" and "starter" fish for that. The hardy ones might make it, the starter ones are cheap to replace after they die. The fish don't usually make it, sometimes they do.
The newer school of thought is to spare the fish that treatment and use an ammonia source, like a dead raw shrimp tied in a bag or knee-high pantyhose as ammonia source. You can also use fish food flakes, they'll break down and become ammonia. There's also janitorial grade liquid ammonia that can be added to tank, but gotta make sure if you shake the bottle, there's no bubbles in bottle. If there are, there's surfactants in it and can't use it in aquarium. Raw shrimp is common, just let it rot In there, but it'll stink.
just know that you'll have to add some kind of ammonia source everyday. That's why raw shrimp is easy, just toss it in and leave it, no daily dosing. But make sure you put it in knee high panty hose, tied off so that when you want to remove it, u don't have to fish out bunch of little prices of rotten shrimp.
I would just get him to your friends tank ASAP in the AM. Good luck!!