When your friend received her first Ledger, she wrote down a seed somewhere. Use that on the second Ledger, and there will be no trace. You cannot transfer the Bitcoins from one to another without a transaction, but you can use the same seed multiple times so you can access the same coins from two separate devices/HWs.
Wait. Using the same seed means the private keys are on the new ledger. But under all new public keys?
The goal is to send the coins from ledger A to ledger B, without being able to trace them.