First baby mama drama

So my boyfriend’s first baby mama is keeping him away from his two other kids they are in another state. (8 yr old boy and 3 yr old girl) she’s blocked him and me from calling them. When I was pregnant with his third (2 months now) we had let her stay with us for a while but all of a sudden she started getting jealous over the fact that her kids were liking it here with us. So she had moved back to her state with their youngest and their son stayed with us from the beginning of the school year until thanksgiving (she wanted to have her son back for the weekend so we let her). Then all of a sudden that Sunday he was supposed to come back she “didn’t have the money” so we tried to get some money to get over there. But we were taking to long and by the time we had the money together she blocked us and proceeded with putting her son in school over there without him finishing the school year here. So now we are worried that he will be held back again because he was held back in kindergarten as well. My boyfriend is soo depressed about this whole situation and I don’t know what to do. I just need some advice on this because I feel like for him to feel better he needs to have them. Oh plus she’s homeless and is diagnosed with bipolar disorder. I worry for the kids because they are stuck in the middle of this and she doesn’t realize that she’s hurting them more than she’s hurting my boyfriend. Last week she got mad that her mom was trying to help him talk to their kids and she called because of that but doesn’t call for them to talk to him. I heard there were bitter baby mamas but this is a little extreme and she’s only doing it because she wants him to come back to her. He’s trying to get all the evidence that supports her keeping the kids away from him. So he can take her to court and get full custody. I mean I too deal with mental disorders and I wouldn’t do anything like this