Angelwings, I am sorry for what happened to the boy you knew. Being close to the situation I know you are anxious to know what happened and to have someone punished. But this is one of those things that can't be rushed. To some extent, you will just have to be patient.
Beginning investigation LE only has the witnesses and their observations to work with. So what the witnesses said is what they go with initally. Then there is the evidence to the autopsy and then forensic testing. Those will be matched with the witness statements. Testing takes time, sometimes days sometimes weeks. Once the results are coming in, then that will be compared with the witness statements- and they better be hoping that the results match their statements. So just because you didn't see an immediate arrest, don't give up. They won't arrest until they believe they have enough evidence to convict.
Here is what interests me. What was happening before. Like SS, I am wondering why she called friends to come and get her and why did they come armed. (Or is he one of those persons who are always armed?)
It sounds like they were all in the car and ready to drive away. Why didn't they? What stopped them, was there something impeding their exit? One of the things that is always looked at as SS said is that it is accepted to kill in self defense- but it is supposed to be a last resort type thing. So even if threats were being made, if they had the ability to exit, they are supposed to do so- not get out of the car and go back to kill him. So you will need to know where he was in relation to the car when he was killed. Was he blocking the car or away from it?
History of the couple will also come into it. Was there a history of DV? Not all DV gets reported to LE, and not all reported DV gets prosecuted. So his friends may know more about any allegations of DV. Under the circumstances they may be reluctant to discuss any past episodes, so watch for phrases like "she accused" "she was acting crazy" and things like that. Then get them to specifically describe the episodes.