Hearing that my friend just lost his mother is making me sad too.
Didn't know how to comfort him. As frankly as I can be, I told him I do not how how to comfort him.
The only thing I can do is by not asking about the details.
If something bad happen to me, the last thing that I want is having thousands of question shoot towards me.
Letting him know I am here if he wants to talk is the best thing I can do as a friend.
I won't say I understand how he felt, because I am not a liar. Only someone who lost their parents will understand how my friend is feeling now.
My friend, you can be sad, but don't be sad for too long. Your mother would want you to be happy.
Take care, my friend.