Consult a L to know your rights and options, but let her do the heavy lifting with the D. If she wants it, she can do all the work to get it. You just be the best dad and person you can be. Stand up for the M.
Remember you both took a vow "...in good times and bad..." Well this is a bad time but a vow is a vow, even if the W isn't living up to her part.
BTW, there are still 7 states that allow dumped spouses to sue the OM/OW for damages. You may be in one of them. If you are in one of them it may help to just bring it up for some leverage if things get messy.
Me:49 W:45 M:19 T:22 EA confirmed and ended 8/2014 S:19,17 D:9,5