Continuing on, it appears that medallion signature guarantees are a US and Canada system. Google AI says it's not available in the UK. Not sure what country your cousin is in.
She may need to come to US and bring a lot of paperwork to prove her identity.
You might call your bank(s) and see which of them offer Medallion Signatures. If there are staff offering this at a branch you can get to, you might ask them if they can authorize paperwork for her without her being an account holder.