I understand your pain. My Mom died exactly 1 month ago today, and I feel guilty for the very same reasons. I knew she'd die from her cancer, but I thought we'd have about another year or so with her, not 3 weeks. I ddin't get the phone call that she was in the hospital--my cell phone ddidn't alert me as it should have--and I got the horrible call at 6 AM that said she was dying, and I had to get to the hospital ASAP, as she had less than an hour to live--if I wanted to say good-bye. She died while we were on the way to the hospital.
Don't focus on your regrets--what's done is done--but think of the good times. That's how your friend would want you to think. After all, even though his body is no longer with you, you will still have each other forever. I don't believe that physical death ends a person's life. He's still with you. Please take comfort in that.