As everyone said this is a legal board. Ya seem like your lost here, so I am going to give you some non legal ideas.
Ya want to help your wife, do like others said, let her vent, rant and rave on you about her situation. When it gets to much for you, and your going to burst and start venting, ranting and raving with mom, stop yourself. Find a way to release it, hobbies worked great for me.
Read boards like this, you will see by reading these posts the mess a 3rd party can make very quickly. You will also learn a lot legally, you will learn what is mom's business and what is not her business concerning the other parent, their time and their parenting style. Read all the posts, today one post might have nothing to do with mom's situation, next week it might, so read them all. Have mom read the boards to.
Read books, co-parenting, stepparenting, blended families, when your done with them, give them to mom to read. Read your state laws concerning custody issues, look up your county see if they have anything. My state and each county in it all have handbooks, they give a run down of the court process, explain terms used, give a run down of what is expected of each parent, how to treat the other parent, what not to do, things like this. After your done reading them, have mom read them.
I even went down to the courthouse for a day, sat in different Judges courtrooms and listened to the cases. I then suggested to my daughter she do the same, you learn so much just sitting there. She did go, and she learned so much by sitting there just one day, listening to these cases.
If you truly do not want to make a mess here, and truly just want to support mom, after reading all this stuff, you will get it and know how to support mom, without overstepping.
I'm not a stepparent, but 7 years ago I became a 3rd party, I am grandma. My daughter and her grandchild lived in my home. I was the one she vented to, and there was much to vent. I was going nuts and wanted to explode and vent right on back to her and with her, but I didn't, I knew that was not going to help her.
So I came to boards like this and read, and kept reading, took me months to even post. I got some great advice when I did first post, from some who happen to be still posting on boards, including this board. They told me do these things I listed above, after you do these things if you truly are in this to not make a mess, truly in this to support your daughter and not overstep, you will get it. They were right, I got it, knew what to do, what my role was, how to support my daughter.
I'm not gonna lie, it's not easy to do these things. You need to find yourself some way to let it out to, be it a hobby or even a therapist. There were times that my daughter would want to vent to me, and I would have to tell her, not today, or find someone else this time. Sometimes you just can't take hearing one more thing, you love these people that are involved in this. So it is much better to be truthful with them, and say not today, or this time find someone else to vent to, I can't take it right now. If you can't take it at that moment, listening to it, can have you doing something, or saying something, you should not.
Best I can tell ya, not the greatest writer and I have a problem with keeping things short, but I hope this helps you as much as it did me.