Waking up sleepwalkers won't frighten them into a heart attack or hurt them in any way, but the tip comes from the possibility of him or her attacking you by instinct. They'll most likely be startled and disoriented when they're woken up, so you might want to guide them back to their bed to avoid scaring them. They're also more likely to hurt themselves while unconsciously walking around, making it the better option to wake them up than to leave them alone.