Back in 2018 i already discovered that his BTC address (created by his Ledger) has funds in it on the BTC chain and the LTC chain, that got me thinking about a derivation problem or glitch.
Have you tried extracting the account extended private key for the Litecoin account which created the address he does have access to, and then using that account extended private key to generate bitcoin addresses? It could be that there was a bug in the Chrome apps causing the Bitcoin app to continue to use the Litecoin derivation path or account extended key by mistake. Ledger have experienced other such bugs with derivation paths getting crossed between different coins:
https://support.ledger.com/hc/en-us/articles/360015738179-Derivation-path-vulnerability-in-Bitcoin-derivatives?support=true