Without access to her Roku account she'll need to link her Roku to a newly created account.
Have her perform a Factory reset via Settings > System > Advanced system settings > Factory reset. This will clear out the unit's memory and she'll then have to set it up again as though it were a new unit right out of the box. During this process she'll be given the chance to link to an existing account or create and link to a new one.
Any premium subscriptions taken out through Roku will remain on the old account and will continue to be charged to the payment method on that account. They will not be accessible on the new account. (If this payment method charges to her, Roku may not be able to help her as they would have no way to determine which of the two parties is the proper owner. She may need to cancel that payment method with her financial institution to prevent further charges.).
Any premium channels with subscriptions NOT tied to the old Roku account will be accessible on the new account via their channel apps.
Channels requiring logins will need to be logged into on the new account.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
If your credit card is linked to "lost" account, call your bank to cancel the card now! Unless you can talk to your ex and have them demonstrate they have deleted card from account (in person or using Zoom, Teams etc in a screen share to see them do it).
If linked to PayPal, log into PayPal and unlink the roku account.