Final answer:
Your friend is likely suffering from narcolepsy, a sleep disorder characterized by the inability to resist falling asleep at inappropriate times. It is often associated with muscle weakness and fatigue.
Step-by-step explanation:
Based on the symptoms described, it is unlikely that your friend is suffering from Restless Leg Syndrome (RLS). RLS is characterized by uncomfortable sensations in the legs during periods of inactivity or when trying to fall asleep, which are relieved by moving the legs. The symptoms your friend is experiencing, such as falling asleep during short car rides, meals, and conversations, as well as sudden bouts of muscle weakness and severe fatigue when excited or stressed, are more indicative of narcolepsy.
Narcolepsy is a sleep disorder in which a person cannot resist falling asleep at inappropriate times. It is often associated with cataplexy, which is a sudden loss of muscle tone or muscle weakness. The symptoms your friend is experiencing, such as falling asleep inappropriately and muscle weakness, align with narcolepsy.
I recommend that your friend consults with a healthcare professional for a proper diagnosis and appropriate treatment options for narcolepsy.