Originally Posted by
Capricorn2112
[ Imagine the look on her face when she realizes that you knew and chose NOT to tell her.
This^. Now is the time to let her know how you feel and that you really think she deserves better. Its bad enough you knew and didn't tell her but if you don't step up now she will likely never forgive you. Do remind her, tho, that you only ever wanted her to be happy. It may take time, but she will eventually realize that you aren't the one who has hurt her. Make sure she understands it was for her sake you kept quiet, not to protect your friend.
Then, step back until she gets out of the relationship with him. You can't be her lifeline on this as Wakeup says. If you do, she will never respect herself and she will end up hating you for it.
Good luck.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh