Sesac also has a website, and you might take a look there, BUT; what makes SESAC believe any of the music under thier 'protection' is being played at the club? I suspect it might well be worth the challenge, because I doubt you'll find open mic-ers singing any SESAC material. Generally speaking, this is a protection racket and the publishing organizations throw their weight around through threats and bluffs. The best way to combat that is knowledge. Take a look at the original material listed as published within the SESAC, take a look at the artists within their fold - you will find that there are a handful of specialty songwriters whose original material is protected by SESAC. They are NOT a mainstream organization, and they are not a folk music specific organization. I suspect it is very likely you could run your open mic for years without ever transgressing the SESAC publishing rights.

I would tell the restaraunt owner, if he must respond, that he should tell SESAC that his venue has never performed SESAC material, and never will without him obtaining a license. They won't like it, and will huff and puff, but would likely be hard ressed to prove him wrong, without setting him up.