First off, I have sympathy for people who get cold sores - I imagine they're uncomfortable and potentially embarrassing.
Having said that, I had a waitress yesterday who had a particularly large cold sore that looked to be weeping (the cold sore; not her) and I suspect she was picking at it whenever she'd go to the back because it was quite inflamed and little chunks of skin that were on it when I first sat down were gone later on.
Fortunately, this was a buffet so I ate my meal and hoped/assumed she hadn't touched the food, but I didn't drink anything she brought to the table as I couldn't be sure she hadn't touched the rim of my glass.
I have encountered similar situations to this a few times dining out, and it is always quite off putting. I'm not sure what the best solution is, but I'm pretty sure somebody with an open cold sore on her mouth shouldn't be serving food.
Does the restaurant have an obligation to keep employees with open sores like this away from food and drink?
Some information on cold sores, including background on how they spread, can be found
here.