This person isn't an anonymous nobody. He writes for a blog that was retweeted by Patrick Harvie of the Scottish Green Party in 2019:
www.google.com/amp/s/www.scotsman.com/news/politics/patrick-harvie-shares-blog-calling-snp-politician-w-1419256%3famp
In the article above, he's quoted as saying the following in defence of calling Joan McAlpine a 'wanker';
"The writer of the blog, Juan Mac, backed Mr Harvie's view and said: "ATF has always aimed to be a consistent voice against bigotry in Scotland, in all its forms. Over the last six years, those from all parties and none have earned Weekly W**r nominations. This week is no exception and the reasons for Joan McAlpine’s nomination are explained in detail on the blog.
McAlpine's role in amplifying anti-trans voices and organisations, while attempting to undermine those who provide services to women and LGBT people in Scotland, has been challenged by voices across the political spectrum, including those within the SNP."
I had a quick look at his followers and I spotted SNP members, at least one SNP elected official, a former editor of The Economist and a person claiming to be an advisor to the Green Party. He does seem to have a bit of reach, so I would understand if JKR looked into taking legal action. I'm not a lawyer though, so I have absolutely no idea if she has a case. I just know that if I were in her position, I'd definitely speak to a lawyer.