i doubt the fact that she used a hardware keylogger.

Hardware keylogger could be easier but I've never has reason to try either, can order them online. The problem with software keylogger is how to get it on there in the first place, for windows there's a hidden admin account with no default password...MICROSOOOOFT! but I don't know if the same is true for Mac. Maybe she found out his login from shoulder peeking.

OP said "all my accounts", so i assume its not all under the same password(?). so peeking over his shoulder is quite unlikely, except he has the passwords written down somewhere.

The login gets her in there, and if he's got passwords saved in browser the login password could unlock them. Probably not though. It'd work on me if someone knew my login, except they'd get stuck on the 2FA for getting into email and I don't save passwords for banking. They'd know all the other passwords though.
Maybe OP has lastpass password set to same as login, the convenience can be seductive.