How to add a second user with SSH access to my Ubuntu PC running Bitcoin Core?
You don't need to grant someone access to the entire machine to be able to use bitcoin-cli. You can provision credentials using the rpcauth parameter in bitcoin.conf (it may be defined multiple times for multiple username/password combinations), and whitelist their IP address for the port you are running the bitcoin RPC on.

They can then use a local bitcoin-cli binary along with the -rpcport and -rpcconnect options to use a remote bitcoind instance.

Naturally, you should avoid leaving unlocked wallets on this server, as anyone with RPC access may spend your coins. It should be treated as a read only instance.