A friend is restless at night but wakes up at 6 am, falls back to sleep and sleeps till 10. What's wrong?

More info. More information is needed to determine if there is a simple explanation or a medical condition. Talk to a sleep specialist.
Depends. Sleep issues are very common and can have many different reasons. If someone snores sleep apnea has to be considered in the diagnosis. A sleep study can evaluate for this as well as problems with sleep cycle abnormalities and sleep movement disorders. Stress, lack of exercise, poor dietary habits and shift work also affect sleep. Your primary can order a sleep study after taking a history.
Sleep interuption. Needs more details but has sleep interuption which can be like restless leg syndrome, pain related arousal, delayed sleep phase syndrome, sleep apnea etc. Try to improve sleep hygiene, avoid caffine intake in the afternoon and if the symptoms still persists then look for more help.