I can't see any way this will be abused.
On the one hand, alerting potential customers to genuine cases of food poisoning is a good thing. You can reduce the instances of people getting sick. Great!
On the other hand, you've now created an incredibly easy way for competitors, disgruntled former employees, or anyone who doesn't like a particular restaurant to mount a libellous campaign of false reports. I understand you review reports and look for multiple instances to see if it's more than just one. But it seems it would be incredibly easy to fool any such system with a bit of coordination between a handful of people. And it's their word against the restaurant's; how do you know who is really telling the truth? (and the restaurant may not even know about the accusation until they find out from your app).
Crowdsourcing can sometimes be effective, but there's been numerous examples of it going wrong, and businesses or individuals being unfairly harmed by accusations that turned out to be incorrect. I hope you've got a pretty good team of lawyers to defend yourself in cases where you get the moderation process wrong, bankrupting a restaurant, and leading them to sue you.
As for a truly negative outcome e.g. a pronouncement by a public health agency that Restaurant X caused a foodborne illness outbreak. This is not possible with a handful of people, as it requires microbiologists, epidemiologists, environmental health experts so on. Unless that handful of people can credibly fool the science, and all these expert participants.
We do have a great law firm, and we are advised as long as we remain a platform for people to report there is zero risk, and this has been tested many times by companies with deep pockets.