Looking for opinions/advice (ask questions if needed)

I (23) have an 8 month old daughter with my now ex husband(25). I divorced him while pregnant because we had an open relationship during the time of conception and had met a woman together (this was not new for us). I immediately saw red flags and I let him know I wasn't comfortable with her. That week after (planned) conception and meeting R (19), I left on a 2 week trip to help my brother move states. When I returned he had let her move in (she was claiming to be homeless but that was later found out not to be true). She was very intrusive and when she found out I was pregnant she became very jealous of the loving attention my husband gave me. She was awful and rude to me and my roommates but she was obsessed with my husband. He was enamoured with the attention she gave him and ignored her red flags. Fast forward, she did a lot of awful things and I decided to give my husband an ultimatum that either he stays with her, and I leave or vice versa. He chose her. I moved out and got my own apartment and went through pregnancy by myself. Meanwhile he was an extremely abusive relationship with her, in and out of homelessness and living in the car I gave him when I moved out. I found out she got pregnant and was due 7 weeks behind me. He ended up escaping his situation with her and got an apartment and a job and was doing good when I was 8 months pregnant he came back to stay with me when I gave birth. Things were great and we worked on a lot of our relationship issues. Until a month later, when she gave birth and she blocked him and denied him any involvement. She said it's not his, refused him any knowledge beyond that the baby was alive. He was planning to try to be involved in both our daughters lives. She was very angry that he was living with me through my post partum that she chose to 'punish' him by not letting him know his daughter there. Now my daughter is 8 months and hers is 7 months. We haven't spoken since the week her baby was born but mutual friends have told us about their concerns of R's ability to care for the baby. For context, R is diagnosed with autism, bipolar, and narcissistic personality disorder. She also claims to have dissociative personality disorder but this hasn't been diagnosed (she has about 10 altars that she claims to host ranging from babies to different genders, etc). My concern is that her baby is severely underweight, as she was born at 6 lbs 10 oz, and now at 7 months is only 12 lbs. As well as from what I've heard from this mutual friend, she is living on cash assistance and has a boyfriend who is 35 and has a history of drug charges. My ex husband has not filed for custody or attempted yet, as his fear is that she will make up false allegations to try to get him arrested or make his life harder. She refused the non court ordered paternity test. She blocked everyone we know from contacting her. He wants to fight for at least visitation if not more in the future if it's in the baby's best interest. I want any opinions or advice on the situation?