We do not know the facts of what really happened. 
Perhaps this young lady was trying to break it off with him and he told her he was going to kill himself if she left him, and pulled out the gun to prove his point,  she tried to stop him and the gun went off striking them both ? 
The most dangerous times for victims of domestic violence are often when they are trying to end the relationship. The other person uses the threat of killing themselves to keep control and power over the other. Sometimes they kill themselves and sometimes they kill other people too.
That this young lady tried to stop him from killing himself (if that indeed is the full truth) she must have been a very caring person who did not realize the lethality of the situation.