I can't think of a way you can honestly be a good friend and not tell her.
I can totally understand the not wanting to be the person to shatter her world but what's the alternative? Can you imagine talking, laughing, being a friend to her now and all the time knowing? How would you feel if a friend took that course with you? - presumably, patronised, humiliated, furious.
The bottom line is that her world is already shattered. The world that's not shattered isn't real. Even if she never found out, the fact is that she's put her faith in a liar and someone who is capable of the most horrible deception. A good friend doesn't leave you in that position, defenceless, not knowing.
If she finds out and finds out that you knew - you'd expect the friendship to be over, I assume.
If the deception continues until he leaves, and you know you had the chance to make her aware so that she could defned herself - get her ducks in a row financially, and you didn't - I wonder how bad you'd feel then.
None of this is your fault. I really feel for you being put in this position. But there is only one option which a friend would honestly take and that is to tell her what is happening in her own life and to give her the chance to take action or at the very least not live a lie.