What should my boyfriend do in this situation concerning his 2 children?

My boyfriend has 2 children (girls, 2 and 4) who he had with his ex girlfriend. Their breakup was ugly, and she wants more child support. Right now, she is being “nice” and only asking for $350 a month. She wrote this out in a document but neither one of them signed it. While I understand this isn’t asking much, she does well for herself financially, and her lawyer/boyfriend also helps her out. My boyfriend is a musician, but doesn’t get income from that, and also does maintenance work which his pay is not on paper (under the table pay). His mom also helps him out financially. She keeps threatening him with court, but hasn’t taken any action. The person who would be her lawyer is her boyfriend. The woman is psychologically and emotionally abusive, has written me letters trying to get me away from him, yet has no interest in being with him. He has his girls every other weekend, she tells me they want more time with him, and when he does go to spend more time with them, she makes it difficult to do so. Also, he raised her oldest daughter since age 1, (who is 6 now) and won’t let him see her now. I know there is not much he can do about that since he didn’t adopt her, but I think it’s a good example that he isn’t the horrible father she tries to make him out to be. He is a decent father who loves his children. We are in Minnesota. Does anyone know what steps to take to get out of this mess? Legal advice?

Here is a sample of her document concerning the money part:

“On the assumption that you have or would obtain a below average job, you would owe: $551.30 for basic support; $246.79 child care support; and $74.00 for medical support, for a total of $872.09 per month. In addition to this child support, I am entitled to reimbursement for all of these items dating back to the first no-contact order for a total of $13,975.53”

I have a copy of her document (which is unsigned by both), have heard her conversations with him, and all the letters she has sent me.

Also, I feel like the only one putting the kids first. She doesn’t send them with proper clothing for the weekend either because she doesn’t care, or maybe to get back at him. He hasn’t been exactly mature about the whole mess either. There are so many variables. What can I do? I’m also not sure if I should take on a parenting role, take charge in this situation, or tell him what to do when concerning his children. Help?