It depends on where you live. Whether or not you can present it as legal evidence, you can however use the tape to document conversations that could be deemed inappropriate and/or dangerous to have with a minor. You build your case and slowly cut him out.
It's sad to have do this, but what's even sadder is that the law protects parents who are mentally ill to the detriment of their children's emotional and psychological well-being all in the name of "respecting parents' rights". Nowhere in the rhetoric is there mention of children's rights to live a sane and healthy childhood.
Ok, /rant off.