By secure folder I guess you mean [https://www.samsung.com/uk/support/mobile-devices/what-is-the-secure-folder-and-how-do-i-use-it/|this]. Theoretically, if you’re not interested in the contents, you should be able to uninstall the secure folder app that was used to create the folder. This app will probably have been renamed to something else to divert suspicion, so I would recommend going through the apps on your phone and deleting any that seem strange to you.
On the other hand, if you do wish to find the contents on the folder, it might be a bit harder. I’ve done some googling, but have found no methods to bypass the lock. If you have an afternoon free you might want to sit around searching a bit.
Honestly, it’s not going to be easy (as far as I know) if your ex doesn’t at least help out. The secure folder seems to be connected to his account, meaning that probably only he has access to it. To open it you’d need something like a PIN, pattern, passcode or biometric verification such as fingerprint authentication. He could’ve stored files, apps, etc on there. Furthermore, if the cloud backup function was enabled it might be possible that he has access to those files without even having access to the phone. If you took any private images, videos, etc using that phone, I’d recommend deleting the secure folder ASAP.
Good luck and Carpe Diem,
Minas