I very much doubt it's her friend. The personality or charisma hardly seemed to match, nor the motivation. Sinon's friend wants to make a girlfriend out of Sinon, I'd say, Death Gun might be another remnant of SAO or maybe someone who wasn't in SAO but dreams of killing for real in a fictional setting (regretting he wasn't PKing in SAO). Sinon would be one of his targets since he obviously wants worthy targets, in some ways, either people who think they are tough or really are, as far as a game goes. Although I do admit that the friend being secretly Death Gun would be perfectly SAO level writing...

I don't think Death Gun really needs to be anything overly dramatic or deep character wise. Perhaps more interesting is how the way to make people die also in RL was developed and whether it was given to DG by someone else. If it was given to him, he can be even more simple, and the real deal would be the source. If that was the case, I hope the source wasn't as prosaic as the what's-his-name villain of the second half of the first season.