Rachael, I still think he should be the one to tell her because he is her son and she won't necessarily believe you. She has to see why w/her own eyes. However, the decision is yours as to whether you tell her or not, but I wouldn't say anything until after the papers have been filed/served. You don't want to tip your hand.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
