Translation:He is a man who has a lot of friends.
The translation at the top of this page is "He is a man who has a lot of friends." I believe Duo contracted "who has" to "who's" in the sentence you saw, which is nonsense here of course but explains the error. I've seen that issue a few times, and the best thing to do is probably just report it. Duo does frequently make odd sentences when it's showing the correct translation after you've made an error (which I assume is where you saw this).
I think the best thing to do is be aware of it, sigh, and ignore it.
As far as I know, this automatic contracting and uncontracting thingy is built deep into the rules of Duo itself, not at a level where course contributors (who are the ones who see reports) can affect things.
So reporting something as "this sentence is unnatural" or whatever won't reach anyone who can help.
"Boyfriend" is usually used to mean "unmarried romantic male partner" in English, and due to societal conventions that situation is much less likely than him being a man who just has many non-romantic friends. What I mean to say is that it seems misleading to translate it as that in English, because a German hearing this sentence wouldn't interpret that meaning from it.
If you specifically want to say that he has many unmarried romantic male partners I would suggest rewording it, maybe with the noun Liebhaber instead (roughly, "lover(s)").
You should generally try to match the grammar of the given sentence for Duolingo purposes (unless it's an idiom or actually makes no sense). Even though "... with many friends" may sound more natural, Duo is trying to teach you relative clauses, so it doesn't accept other structures.
Yes, "who's" (wrongly contracting "who has") is a common issue, but unfortunately is a difficult error that can't be fixed easily or soon.
That's a perfectly good translation, but Duo is trying to teach you relative clauses and won't accept answers phrased differently. For Duolingo, your answer should generally match the structure of the given sentence even if it sounds a bit odd, as long as it's still grammatical.
Duo is trying to teach you relative clauses here, so it wants you to translate with a relative clause ("who has many friends"). I do think your translation sounds better, but in general I recommend that you avoid rephrasing Duo's sentences in your answers if you don't have to. A lot of the time Duo expects the specific phrasing it gives in its prompt.