We're only seeing the event in question from one point of view, and the guy is certainly not going to portray himself in a bad light.
I've worked in numerous customer service jobs over the years, generally mid-tier establishments (ranging from hotels, cafeterias and delis) and the one thing that kinda got my alarm bells ringing is that there's no mention of how the customer treats the staff.
In my most recent job we had a number of returning customers that we all generally disliked. I recall one guy in particular who was incredibly patronising to all male staff and sleazy with the female staff. He never crossed the line into outright verbal abuse or sexual harassment, but he was problematic as there were times when it would extend to other customers in the shop. An extension of this would see him trying to do special orders which we would just flat out refuse, as the way he treated us left a lot to be desired.
In many instances behaviour makes a difference. Customer entitlement is a very real thing, and the lack of basic manners is a dealbreaker with me. One time there was this regular customer that was buying soup (we have a self-service area for that) but had grabbed one of the mid-size coffee cups instead of the soup cups. I simply pointed this out to him and he said that he knew, and that he was helping himself to a sample. The brazen nature of this got to me and I said that we don't do samples, at which point he threw a huff and loudly exclaimed that he always has samples and that he's never seen me working there befre. I had been working there for eight months. My issue here was that he didn't bother to ask a member of staff beforehand, a kindness that was extended to him in the past was now being treated as standard procedure. I do actually accomodate some special orders, but I'm often careful and exclaim that this is a special circumstance and should not be thought of as usual policy (and often this has to do with customer demeanour, I will absolutely stonewall someone that acts a prick and demands something not otherwise listed). An extra kicker in this instance was that there were espresso cups available if he was intending on sampling, grabbing a medium cup is the action of a chancer.
To bring it back to the OP though, the guy is a businessman that is wining and dining a potential client. I have never had a good experience with this kind of customer, a big part of the wining and dining is impressing the client, and often that impressing is done by demeaning and belittling the service staff. Make yourself look like a big man and all that. Any reasonable person would accept if a special request cannot be done outside of dietary/health reasons, to push the issue is a sure-fire sign of the guy trying to flex his muscle in front of someone he's trying to impress. When that's the case, I totally get where the manager would be coming from. You don't necessarily want to lose the patronage of a regular customer, but I doubt that customer is highly valued.
We also have no scope for the success of the restaurant. Taken out of context $500 a month sounds like a big spender, but we have no comparison to the average takings of this establishment. For all we know he's actually on the low-end of regular customers, in which case I would sympathise with the staff even more. In every job I've worked in there have been regular customers that want the place to accomodate them when in reality they spend little compared to others (that first man I mentioned exemplified this). We've had customers that can come in for lunch every day, drop twice as much, eat in half the time and make a fraction of the fuss.
As everyone else has said, go somewhere that will accomodate you. Don't kick up a fuss at the place that doesn't.