I really shouldn't get involved in the tiff, but might I query what might possibly constitute defamation that can be proven?
Or to ask the question a different way, what are you worried about? If there are people here who believe what he is saying, then that is a fair safety net. Secondly, in what way is any opinion from this website about a player, coach, official, coach (etc) able to cause serious damage to a reputation? No offence intended, but this isn't a place for serious sharing of ideas...
Thirdly, even if 1 was false and 2 somehow proven, wouldn't the individual be liable anyway?
As you suggest, he is just sharing an (unsourced) opinion on a footy forum in a rumours thread, where the general tone (for hundreds of pages) is people slagging off opinions left and right and havubg heated arguments. I truly don't see much chance of a case of that impacting anyone outside of the forums reputation in a substative way....