Translation:I respect every human who is a friend of animals.
Why? Because we're very unlikely to use "human" in a sentence like this. What we'd probably say is "anybody", but forced to use a noun our first choice would probably be "person" (which I now think you accept elsewhere), and possibly "human being", but I can't think where we'd ever talk of "a human".
The człowiek/person/human etc. distinction is a pain in the... head on a website which relies so much on word-to-word (if possible) translation. "człowieka" sounds perfectly fine to me. "osobę" probably as well (here), because it's some hypothetical person, but it was not the choice of whoever created this sentence. So I shouldn't put "person" as the default version. Is "human" really so bad here that even "human being", which sounds very... scientific is better?
Well, I'd never refer to "every human". My first choice would be "any/every person", followed by "human being", which I agree is a bit strange, but it's a lot less strange than "every human".
The first definition of człowiek in PWN/Oxford is "person", followed by "human (being)".
The word "human" is usually an adjective, and when it is used as a noun, it is almost always used in the plural. From Merriam-Webster's Learner's Dictionary:
human (noun): a person — usually plural
And at the Free Dictionary - human
2. A person: the extraordinary humans who explored Antarctica.
I've checked eight dictionaries and I can't find a single example sentence where "human" is used in the singular. Oxford has 19 example sentences, all in the plural, where "humans" is usually used in a general sense, for example:
There is every reason to think that you would come across problems cloning humans.
In those days, sailing solo meant a form of isolation that few humans could endure.
The tsunami may be an act of nature but humans are complicating the relief effort.
You could perhaps say,
"I respect all humans who are friends of animals"
but it's getting a bit far from the grammar of your original sentence.