What a tragedy. It sounds like you are having a hard time as well as his mom, which is completely understandable.
I haven't lost a child, but I have friends who have and there are a few things you might think about doing. First off, in the immediate future is to think of specific things you might do to help your friend. By this I mean, don't just say, "I'm here and let me know what I can do to help." When a tragedy like this stikes, the person left is often incapable of thinking what needs to be done. Instead, suggest things like, helping clean the house, take the car to the carwash, trim the yard. Tasks that the mom might not be up to at this time. Of course, it's also important to let her know you are willing to help out in whatever ways she might need, but always make suggestions first.
Many people feel uncomfortable talking about the person who has passed, but usually doing so is incredibly comforting to the person left behind. If you are alone with her, ask to see photos of her son. Ask her to tell you some stories about him. His memory will only be kept alive by sharing who he was.
Regardless of how long it takes, stick with her. Too many people think that others should "get over" their loss much too quickly. Even if she comes back to work and seems "normal," don't assume that she is. Continue to offer help on an ongoing basis for as long as she seems to need it.
Tell her how grateful you are to have known her son, the role he played in your life. Often moms aren't aware of this and it is very comforting.
Finally, don't neglect your own feelings. Don't be afraid to cry with her. This is just one more way to share the loss that can be very meaningful for both of you.
You're a wonderful person to want to help and support this woman in her loss.
I wish you well.
Warmly,
L.