You are right to worry
. There are several problems here--the risk of heavy alcohol use, the risk of a serious substance abuse disorder, and the risk of alcohol dependence.
Heavy alcohol use to the point of blacking out is very dangerous because can lead to physical and sexual abuse, criminal transgression, and exposure to unsafe environments (very cold, operating machinery, insufficient caution around water and heights). As you probably know, very high alcohol levels can cause people to stop breathing or have serious heart problems.
Blackouts are a sign that a person is unable to moderate their drinking, which is an important sign of alcoholism
. Alcoholics are unable to stop despite major adverse consequences for their life, such as failure in school, estrangement from family and friends, serious health or safety concerns as described above, and legal problems. They often try to cut down but are unable to do so.
Finally, heavy daily drinking (more than 2-3 drinks per night) can lead to physical dependence, in which the body becomes so used to alcohol that it is less able to function without it. Symptoms include tremor and irritability when not drinking, but can progress to a very serious condition causing seizures
, confusion, and death.
If you or your friend are concerned about alcoholism, it is very important that you see a health provider who can evaluate your concerns and recommend specific treatment.