Accidental stolen kitten - return or not?

[Yumdoritos]

Bit of a weird one -

I was visiting my friend abroad, he lives in a small town. We were in the park there and a tiny kitten ran up to us, rubbing itself on our legs and playing around us. It had no collar on and looked really young, maybe 3-4 months, they shouldn't be outside at that age?

Anyway it was running around without a care in the world, could have easily run into the road as it had no road sense and was super confident and friendly. We took it to the vet to check for a chip. It didn't have one, but it had bad fleas. Wasn't neutered and the vet said it wouldn't have been vaccinated either, so assumed it was a stray

We paid for flea treatment and worm treatment, and unsure what to do with it (there are no shelters nearby) got it microchipped to my friends details and planned to keep it as a house cat. Took it home, fed it, it ate LOADS. We named it and understandably got attached to it.

I am back home now in a different country, but my friend said that today there were missing kitten signs around the town.... describing the kitten and saying it hadn't returned from being let outside

Should my friend keep the kitten or give it back to its 'neglectful' old owner? He paid for its medicine (flea and worm) and chipped it to himself. It was outdoors unvaccinated, unchipped, unneutered, and at way too young of an age (3/4 months) AND covered in fleas, I'm not sure that the kitten should be given back to an irresponsible home like that, but we both feel guilty for accidentally stealing it. What's the right thing to do?

I'm dubious over the "saying it hadn't returned from being let outside", it seems an odd thing to say when a cat has been lost. It might have escaped.
You don't know about the vaccinations, that was an assumption.
The fleas could be a concern but you don't know how long this kitten was missing before it was found.

The poster is in the language of the country this is in so it would be useless to put the picture of it on here - but my friend translated it as something like " hasn't come back" or something which worded it like it had been let out before

It's the vet who made the unvaccinated assumption, and well... he is a vet, so we obviously trusted his opinion. Naughty kitten has smashed a glass today according to my friend so he may be sending him back after all!(joking) I told my friend it's his fault for leaving things out that the kitten can break he sounds like he is quite happy living with my friend though

If people think the phone call thing is a good idea then I can nag my friend into doing it, just wasn't sure if it could get messy if they try calling the police or something. But he's chipped to my friend and my friend paid for his treatment at the vet so not sure what the law would say about it? Friend has had him for 2 weeks or so now

I know my replies come off like I am trying to justify everything but it's all true! If my friend wanted a kitten he could have got one far less stressfully than this way he also has 2 cats already so he didn't just steal this one on a cat-craving whim!