In the event of someone having a seizure, they shouldn't necessarily be left alone but they shouldn't be handled, either. They should be supervised to ensure they are not hurting themselves. Someone should call 911, first and foremost, as the Paramedics have medications that can stop the seizure.
Secondly, if a person is seizing and is making motions that will hurt them (i.e. banging their head on the floor-unintentionally) someone should get a pillow and use their hands to support their head to prevent them from doing so, as a head injury could further complicate a seizure.
Lastly, never stick ANYTHING in a seizing person's mouth. Swallowing the tongue is not possible, as old myths suggests. They may turn blue if the seizure is prolonged. This is a normal variance because the seizing person cannot breath during a seizure. So, making sure 911 is called first is the most important part other than to making sure that they're not hurting themselves.