Ex-Client Is Threatening to Sue Me Over Failing Business, How Do I Tell Him To F*** Off?

So I had, or should I say, had a client who used to be a pretty nice guy, or at least I thought so until recently when he messaged me saying I was tampering with his business and how he can't think of any other reason his roofing company is now failing other than it being my fault and how I should expect a lawsuit or to hear from his lawyer in the near future.

I haven't said anything back to him despite having a pre-F*** off message in my drafts, but to give you some context, it wasn't always like this. I met him through FB when he messaged me after I posted some Google Business Results from another client of mine, and after some talking back and forth, he agreed he wanted to give it a go. Things were great, rankings were moving, client was very responsive. Essentially, everything I could ask for until eventually he decided to go against my word in favor of a buddy he knows. Whatever right it's his business, but this eventually became a constant theme and would leak over into our work. Before you know it, these "Suggestions" his buddy was making would move our rankings from number 2 in the map pack generating 50-60 calls a month down into the top 5 where calls were still good but nowhere near the same, and you could feel there was no bottom/end to this until eventually he fired me when I refused some of the vital changes he wanted.

I didn't take it to heart, aside from the friend stuff he was a good guy so we parted ways but I guess things continued to drop in ranking once I left. Now fast forward to a few months out and I guess he's convinced himself that I'm doing voodoo black hat magic against his business. Like I said interesting scenario but I can asssure you I'm not tampering or doing anything and it actually pisses me off a little that he would assume that, considering my only goal in the first place was to help him out and I even gave that little rat a discount when he first signed. I'm a little heated but what do you think the best way to tell him to F*** off is without escalating this any further?