Yes, the waiter is 100% wrong and the restaurant should have fired him immediately. In addition, the couple getting “the law involved” are wrong as well. When we can’t handle these types of things ourselves, we are doomed as a society.
Spitting in food, or worse, is tampering. You want some HIV/Hep-C case spitting in your food? Had that lid not come off, the customer would have consumed the drink, ingesting who knows what besides just saliva. Hep-C isn't funny, and if left untreated, deadly.
Damn right I'm gonna go after someone who tampered with my food, especially if they look like they putt from the rough.