Well, this is bound to be the least funny advice column this Funny Guy's ever written, but here goes: as misfortune would have it, a dear friend of mine died a year ago this month as well. In fact, he was the first "dead person" I've ever known (yes, I've been that lucky). The first funeral, the first unbelievable phone call, the first shocking absence.
And I learned something from that experience: grief is a hairy motherfucker. Death is something we all have to face, and in a lot of ways it's beyond our normal modes of comprehension. The effects it has on us are varied, and it's not our place to judge anyone's grieving process, unless their grieving process is to hurl typewriters at laughing children or something.
Your friend's friend and his girlfriend had to go through the same process you did, and they've learned their own lessons, been affected in their own ways. Is a year too soon? Five years? Ten years? Who's to say your dead friend wouldn't WANT his girlfriend to find happiness and solace with someone he valued so highly.
Let it be. In the face of death, love is all we have. I'm absolutely positive that these two mean no disrespect, or you wouldn't have called them "his girlfriend" and "his best friend." Unless you think they plotted his demise, then they're just bystanders trying to make sense of it all, the same as you.
I'm sorry for your loss, and for the paltry laughs in this post, but I thought this was something worth discussing. Life is short and often arbitrary; begrudging someone else's happiness will just make it seem all the shorter.
The dead want for us what we want for those we will leave behind. The dead want us to live. Live.