Look up lifespans of indoor and outdoor cats. It's pretty dramatic.

Cats outside are an aggressive invasive species in every continent except Africa. I love them but they need to be kept inside, and neutered/spayed. Cats do fine indoors, even ones who have gotten used to going in and out. They may complain but with enrichment and regular play they will get over it. If not, then a catio is a good solution.

My family used to have the view that keeping cats indoors was cruel until one of our indoor/outdoor cats came back after being attacked by some unknown animal in what seemed like a safe suburban area. He was fully vaccinated and recovered mostly without incident, but he had to have a surgery and spend extensive time in the Cone of Shame with limited movement so he could recover.

And once we started keeping the cats indoors? It was unbelievable how much nature outside recovered. It was more than just songbirds everywhere (though we did have plenty of those). We saw hawks and owls. Woodchucks and deer would wander in (yards weren't really fenced at this area... it was more on the rural side of suburban). Foxes and rabbits and wild turkeys. I once saw pawprints in the snow that I think were from a stoat. We could not believe how visible a difference just keeping the cats inside made.

And honestly, the cats didn't really miss being outside after a week or so adjustment period. Once, a year or so in, Mom accidentally left the door to the outside open, and one of our cats just got this resigned look like "oh, I guess I have to go patrol the area, huh."

Now, this was a two-story house with like five bedrooms and a basement, so it was more territory than, say, an apartment, which can make a difference. We also had two cats to keep each other company, so the house being empty during the work/school day wasn't as big a deal. Space concerns are a bit tougher to address (though I will recommend catification, catios, and leash training for cats to do supervised outdoor walks), but if the concern is a cat getting lonely during the work day, then I'm just saying that two cats are not appreciably harder to take care of than one and after the introduction period they can do wonders for keeping each other entertained.

Your friend is a bad pet owner.

If they don't think animals should be kept in captivity, they shouldn't keep a pet. Pets are, by definition, captive animals. If I befriended a pigeon by feeding it, it wouldn't become my pet; it only would if I captured it.

Let your friend know that most experts agree:

Outdoor cats are devastating to the local ecosystem, and have a significantly shorter lifespan than indoor cats.

Did the cat choose of it’s own free will to come live with your friend?

If not, your friend is already responsible for a domestic animal they chose to acquire. That comes with responsibility. Domestic cats aren’t compatible with the outside environment and cause ecological damage.

Cats are apex predators, and your friend is subsidizing their pet far past what the local bird population can support. (Plus indoor cats live longer.)

Unless you're on an actual working farm, household cats belong indoors.

Well, this is the consequence of letting it outside unfortunately- on top of risk of injury and death but I’m sure y’all know that already. There’s not really an effective way to train them against this behaviour the way you might a dog.

So long as there’s critters outside the cat is going to hunt them. Maybe you could try to give the kitty a snag safe cat collar with a bell? It might alert the wildlife so they have time to avoid being caught.

I'm afraid you've already lost that battle. It's their home.