If boyfriend was cleaning his gun and bullet went off and killed a friend what will happen to him?

The answer depends heavily on the very specific facts of the case. Would the authorities have any reason to doubt that the gun's going off was entirely accidental? Was your friend exercising the care which a person ought always to use while cleaning a gun? Assuming he was careless (as seems very likely) then he could be charged with one of the offenses charging reckless endangerment or otherwise endangering another person's safety. If the court believes it was an accident and your friend is genuinely remorseful, the penalty might well be light, such as pure probation.

He will likely be charged with some type of negligent manslaughter.

He would likely be charged with involuntary manslaughter.

A number of things could happen. The friend's family could sue the boyfriend(BF) for wrongful death. However, if the BF has no insurance coverage for this (such as homeowner's insurance), and no assets from which a judgment could be collected, then the family would have little to gain by filing a suit. He could be charged criminally. The most likely charge would be involuntary manslaughter, which causing a death by a reckless act. I think it carries up to 5 year (in S.C.). However, if the authorities determine that he was merely negligent but not reckless, it would not support a charge of involuntary manslaughter.

From a civil standpoint, the person cleaning the gun who somehow accidentally shot another person while cleaning the gun is going to be financially responsible for the medical expenses, lost wages, and pain and suffering he cause to the shooting victim. It is up to the District Attorney to determine if there is a likelihood of proving the person who was cleaning the gun had an intent to shoot the gun before deciding if there is a basis for a charge of assault with a deadly weapon charges or attempted murder charges or any other crime will be filed against the person cleaning his gun.