What a terrible position to be in...........I really feel for you. Do you have any idea why her family has not disclosed this horrible fact to your friend yet?
You don't say how old you are, but the best advice I can come up with now is to speak with your mum about this. Tell her your concerns..........perhpas she knows why the family is witholding this information.
I don't mean to say anything against your mum, but I think it was a bit unfair of her to tell you.........knowing you couldn't say anything to your friend. That is a great and difficult burden for anyone to deal with. Ask her how you should handle carrying this terrible secret around!
If you can't talk to your mum about this, is there another trusted person in your life you can turn to? An aunt, a grandparent, someone from your church or school?
I'm very glad you found us and I hope other's here will have better words of wisdom for you. You might also want to go to the Cancer Forum and post this same message there. I'm sure you're not the only one who has ever been put in this very difficult position. You will also find much good advice on how to help your friend when she does find out and how to help her on this very sad journey. She will need your support and understanding. I wish you the very best. Please keep posting here anytime you need to talk, OK?
Peace
Greenlydia