A few preliminary thoughts: to me no fries would have been a deal breaker, indeed if I'm eating a meal with fries, I'm pretty much willing to share anything EXCEPT the fries. But actually asparagus are good too (but not as good as fries...better than burgers though
Anyway, yeah I would have paid with a credit card instead of a debit card too. I'd probably also have tried to negotiate something else with the restaurant manager such as vouchers for however many fries should have been in the order that I could have passed out with the burgers to the people at the event.
Also, one key piece of information the story is lacking is whether or not Bob was, or expected to be, reimbursed for the food. At first reading I somehow assumed this was for an office party or club event that was going to reimburse Bob. Upon second reading, it seems like the party may have been a private party with Bob paying for the food himself, with no expectation of being reimbursed, because he's the one hosting the event and/or supplying the food is his contribution to the event. If it was an office/organization party, I think Bob should still be reimbursed, and if he was, I think he should just drop it and let the office/club absorb the cost.
If he wasn't reimbursed or didn't expect to be because it was a private event, then I think he should complain to the restaurant's corporate headquarters. Even though it's an independently owned franchise, depending on the restaurant/franchise, the restaurant could still lose its franchise license if it's making the whole chain look bad. Also, the story said 'restaurant manager'; was the manager also the owner? If not then Bob should definitely take it up with the owner too.
I think if it were me throwing a party and buying $400 worth of food, and only receiving $300 worth, I'd probably just accept the loss. I would expect to spend a lot on a party and an extra $100 wouldn't disappoint me that much. $500-$1,000 sure, but $100 I'd just shrug off as part of the cost of the party...plus really it was going to be spent either way, it's just the guests didn't get the food. So I'd focus more on making sure the guests still had a good time and apologizing for the lack of fries. Complaining about it incessantly and keeping CarefulBuilder from enjoying his burger just makes the situation worse.
I'd also perhaps look for a creative solution to the fry shortage. $100 worth of fries couldn't be that many orders of fries, maybe 20-30 orders? I'd see if I could task 4-5 willing friends to go to 4-5 different restaurants and order about 5-6 orders of fries each. That's not so many that it would likely take the restaurants that long to get them together. Of course it would increase the cost of the party.
Regardless, if the restaurant didn't provide fries vouchers and/or take off the cost of the fries, I'd stop doing business with them and make sure that my guests also knew about the problem so that they could make up their own minds whether they wanted to eat there in the future.