My friend sleeps with eyes open. what causes it and what's its solution?

Eyes: This condition is called legophthalmos and may lead to dry symptoms due to lack of a tear film overnight. It is caused by improper function of the orbicularis muscle in the eyelid for one of many temporary or permanent reasons. It is treated by addressing the underlying problem, and possibly by using a prescribed eye gel at night or mechanically taping the lids shut.

Taping?: I assume this is a very close friend or you wouldn't have observed this! Often the cause is sleep position, especially sleeping one one's back without a pillow. If your friend's eyes are open when sleeping in a sitting position, I would be concerned. Narrow strips of PAPER tape connecting the upper lids to the cheeks at bedtime can keep the eyes closed and prevent irritation and dry eye.