That sounds ridiculous. Firstly, none of the riders were hit. Secondly, anybody riding a horse assumes the full responsibility of their horse - if the horse gets scared of a car and falls off, that's the owner's problem (Unless the car deliberately scared them). If you're riding on the road you should be prepared for anything that happens. My horse happens to be terrified of moderate traffic and throws a fit - does that mean I can sue the drivers of all of those cars? Nope.
At my barn, there is a sign saying something along the lines of "Any person who practices equine activities takes full responsibilities for injuries caused by practicing said sport" Or something like that. And it's completely true. If he showed proper courtesy, which he did, they shouldn't have any grounds to sue. By riding on the road, they are accepting the risk that their horse could get scared.
Also... how on earth did they go to a doctor in fifteen minutes?! Even if they sue, I doubt they have any grounds to get anything out of you. Of course, I'm not a lawyer, and talking to one would still be advisable.