Tell your mum what happened and have her call her bank and report her credit card as lost. A new number will be issued so Digital Ocean won't be able to bill her card any more.

Also contact Digital Ocean via email and explain what happened and that you need to close the account. Then you have a legitimate record of what happened so if they try to send you an invoice, you have evidence that they shouldn't be doing this per your earlier request.

Good luck and I'm sorry to hear this happened to you.

Contact DigitalOcean support to have them close your account. Maybe someone on DigitalOcean is listening in here, but it’s better to try all channels.