This is a difficult question to answer, as the choice is primarily one you have to make based on the moment. It is usually best to speak to the person with epilepsy about what they would prefer that you do when a seizure occurs.
However, regardless of this conversation, if any of the following points are applicable, then you should call for emergency medical help:
The seizure lasts for more than five minutes The person has no history of epilepsy, as far as you are aware, and has never had a seizure before The seizure occurs when the person is in water (e.g., swimming) The person is struggling to breathe or walk after the seizure A second seizure occurs rapidly after the first The person is injured during the seizure The person has an underlying medical condition, such as heart disease