You won't have been told the full story, that's for sure, OP.
If you attend hearings with him, and the final court hearing (not sure if they allow that?) then I'd advise you to keep your ears open, and be alert to the fact there may be some truth in his ex's side of things. As the whole story comes out, you might revise your opinion.
We had a very lengthy, traumatic journey through the Family Courts with my ex a number of years ago, and the upshot was he was one of the less than 1% of men who was denied any direct contact. The courts didn't come to that decision lightly. They bent over backwards infact, to accomodate my ex but it became increasingly clear he was a nutjob. Had he had a new girlfriend in tow, I have no doubt she would have been loyal and seen me as the villain.
Why do you think the debts were purely her's? Where was he when that happened? Did he leave her short of money to care for the child? Did she have an unusual level of dependency on him that left her vulnerable?
I'd be a bit more open minded. She will be the one picking up the bill for the kids no doubt (My ex paid not a penny in maintenance, never has, and the CSA claimed he didn't exist and they couldn't find him. Took me under 1 second to find his full address on Google, and little more to find out about his exciting adventures on permanent holiday in Italy, on FB).
Even if he does pay - it won't come close to what it costs to raise a kid.
Is spending time with him even healthy for the child, if he is so negative about the child's mother?
Start asking yourself hard questions. I'd trust the court process to make the right decision. I'd use this time it's going through to drill down on the reality of the case. Rather than the self serving version your partner appears to have told you.