Should I tell her the truth?

Good pm ma, please ma I urgently need your advice. I will go straight to the point.
My younger sister just found out that the man that she has dated for up to two years now and he even came to seek for her hands in marriage was a married man with three kids. She got to know cos I wasn't in support of him marrying d man. My spirit never accepted the man right from day one.
I advised my younger sister to enter school first before talking about marriage but when it seems that all she wanted was marriage, I advised her on making necessary enquiries about the family she wanted to enter and in the process, she just told me today that she found the pictures of the man and a woman in facebook.
She went ahead and gave me the woman's facebook Identity, then after chatting with her, I found out that she has been married for five years with the man.
I told my sister about it and we both decided to seek advice here on how to handle the issue.
She has already decided to quit whatever she has to do with the man but the problem now is what to tell the woman cos she has been asking questions because of somethings I have asked her.
Should I tell her the truth?

Dear sender, 
Thank God for saving your sister from the hands of an unfaithful man. 
You do not owe his wife any explanation to avoid wrecking her home and putting her into unnecessary pressure and worries. 
Simply diffuse the discussion by telling her that you cared to know more about her or better still block her off from your account. 
Maybe this will sound as an experience or warning to your sister that not all who appear single are truly single in every sense of the word. 
Let her terminate every discussion or communication with the man and if possible change her contact to avoid harassment from him. 
All the best.