Open up the instruction book to any game in your collection, and read the disclaimer. It should say something about Epillepsy. It's been proven that the screen's refreshing at a constant, but sporatic, rate, can aggrivate epilleptic side effects in people who suffer from the disease, even if mild. For some people it's a mild headache, for others it's passing out with foam at the mouth, for others, it's the head fully imploding and causing a tear in the space-time continuum (okay, I made the last one up). But tell your friend to read the Epillepsy warning in the manual
"In an interstellar burst, I'm back to save the universe"