Can Confidental Medical/Therapy Records be Brought into Custody Dispute?

You can't steal confidential records to put before the judge. What you have to do is allege that there are mental issues which affect you're partner's ability to be in custody, and ask for a court appointed psychologist/psychiatrist to ess your partner. Then the judge will probably adjourn the case until he gets the information back from that essment. If your partner refused to attend, it would reflect very badly and give you more chance in court.