You might want to talk to your friend and tell her that you are concerned about her health and her baby's health. Offer to tell her Mom or to help her tell her.
It's good that you talked to your Mom about it... Moms usually have good advice.

I know what it's like to be in your position, and it's hard.
Here are some things that you should think about...
1. Is not telling her Mom or going to a doctor really endangering her life?
2. If she is over 18 and you have given her good advice, it is her responsibility what she does.
3. I know that it is hard to stand by, but would telling her Mom really change anything?
Does she have any health problems that make her pregnancy more risky than normal?
Why hasn't she gone to the doctor? If she is 20 I assume that she is the one that takes care of making appointments for herself and not her mom?
Really, I don't have any advice... You're in a really hard place. I'll be praying that God would give you wisdom of what to do.