Wait a minute. You said that the coins were visible in an address when you imported it into a watch-only Electrum wallet. So why don't you try playing around with the derivation path on both seeds to change it to ones that could be probable Trezor paths? It's basically the first 3 numbers you need to manipulate. It goes down to the second and third numbers if you know the address prefix which you seem to know: Paths start with 44, 49 and 84 (I believe they are all hardened) for 1, 3 and bc1 addresses respectively.
He knows the bitcoin address lol. i the question is, did his sister or whoever she was how did she get that address into her trezor?? i doubt she would know how to create a watch only wallet and even if she did know how, why would she do that and where would she get the particular bitcoin address from to do that? the mystery deepens...