This is indeed a breach of your intellectual property right/copyright in the plans and as such you are entitled to compensation and that compensation is likely to be whatever it would cost to have the plans drawn, plus the cost of any court costs.
The fact that they were available from the local authority planning department website is irrelevant. Just because it’s on the Internet doesn’t mean it’s in the public domain.
The Internet is full of images which are freely available to look at but you can’t use. I deal with dozens of claims every year from people who have used Internet discovered images on their website and in their advertising blurb only to get a bill of thousands of pounds from the image owner, Getty Images, or suchlike.
So the fact that he has got them off the Internet is irrelevant. He needs to take better legal advice.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have