What happens to ETH after sending it to an invalid address?

I tried googling around but got multiple obscure answers such as either it being lost forever or returned if the exchange allows it and it really depends on the code.

A friend tried sending some to my address but the input somehow got truncated by the last letter. I see the incorrect address in their logs and the money is gone but etherscan does not recognize it as a valid address.

Checking from their address, etherscan does not see it as well.

I understand if it were a valid but incorrect address (wrong hex,length,etc) but what happens if it doesn't satisfy all requirements to be a valid address?