Anyone had a way to help someone in prison access their account or get third party mandate
Prisoners are not going to be able to have access.
If they let someone on the outside do it for them then that’s their call.
The account holder would need to contact Monzo.
If you email firstname.lastname@example.org and explain who’s gone to prison, along with their address and email if possible, one of their specialist teams will respond to let you know the next steps and potentially send you a form to be completed.