The benefits of running far surpass just the physical ones (like fat and weight loss, lean and toned legs, increased cardio vascular endurance, and improved heart and lung health). Numerous studies have linked running to having a positive impact on the brain and serotonin levels in the brain that normalize emotions and moods. Running and exercise in general is often prescribed by doctors, counselors, and therapists (often along with medication and therapy) as part of a healthy routine to fight depression, ease anxiety, and maintain a healthy level of stress. Running also helps people with ADHD and OCD because it helps release excess energy and helps create a more ordered structure to their routine.

For some, a little bit of exercise (like running) actually may help decrease epileptic episodes. This is believed to be true because of how cardio vascular exercises promote an increase in blood flow and oxygen throughout the body, and help reduce stress. (Epileptic patients are also at an increased risk for developing high blood pressure, type 2 diabetes and depression and anxiety, and because running has been linked to helping lower the risk of these illnesses, running could help decrease their likelihood. Particularly important for decreasing the likelihood of a seizure is to keep stress low, and running is one of the best (free and healthy) ways to reduce stress. However, doctors lately have been warning against running if you are an epileptic patient or someone prone to seizures. This is especially true if whatever is causing your seizures has not yet been determined.

Running may reduce stress, but too much running actually creates excess stress - especially physical stress. And if the individual becomes obsessive or overly concerned with his or her running, then making sure they get in their run each day for how long they want to creates even more mental stress for them to deal with... which greatly increases the risk of a seizure in epileptic individuals. This could have been what happened with your friend. Whether it was mental or physical stress they were under, it sounds like running wasn't serving to help their serotonin levels as much as it was causing their bodies to be under too much stress.

Sources:
https://www.epilepsy.com/living-epilepsy/healthy-living/fitness-and-exercise/benefits-exercise