I would say its about trusting the source you got it from. There are many times I've tested files that have flagged a virus that I knew infact did not contain one, by AVG, and Avast and most of them can get a 'false positive'.
I infact am sharing a file now that has flagged some ppls antivirus but its definitely not a virus.
That said you need to trust the source, have they checked/used the actual file? Did they get it from someone and just believe its clean when infact its not? Those are things you have to take seriously, Im sure this person believes (or knows) the file is clean however things can happen and if your not sure its best to get a new source and file. Its not usually a case of the av companies trying to thwart file sharing as much as they often flag up certain parts of files as a known issue when infact its not the same like keygens or for example a while back AVG had the installer we supply listed as a virus due to the script used to build the installer, and it was a false positive.
Always use your best judgement when dealing with these types of issues, if your unsure its best to try a new source.