(((( HUGS )))) - Yes this is indeed a tough one, but it is one that needs to be dealt with ASAP - so it does not get in the way of your relationship with your mother.

I personally would tell her...... just stick to the facts and keep it clean and straight so you do not create any animosity or upset in your mother - she is going to need some time to process this before she can rationally cope with it.

Take care of your self and stand strong....... do NOT let this man run over you, he is a jerk with a capital J.