Translation:He is a man who has a lot of friends.
The given english translation is grammatically incorrect. "Who's" is simply poor. If anything, "whose" might suffice, but as there is no context in which to place the German phrase, a better answer would be "He is a man with many friends" as it is more generic translation.
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)").
"who's" is totally incorrect. "He is a man with many friends" should be allowed!
If the sentence means 'he is a man who has many friends ' then 'he is a man with many friends' should be an acceptable translation. He is a man who's many friends is not an English sentence.
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.
A German word for who is not there so how can who be there in the translation