Adding additional SSH key to existing Droplet

I have a droplet with no-password ssh access. I would like to provide access to my colleague.

I had him generate a key-pair on his linux machine, and got his public key. Then, I added his key to the ~/.ssh/authorized_keys file along with my key, following the tutorial here: https://docs.digitalocean.com/products/droplets/how-to/add-ssh-keys/to-existing-droplet/#manually

I did set the permissions afterwards as instructed.

I’m still able to ssh in, but he cannot: Error Permission denied (publickey)

I tried service sshd restart, but to no effect.

Similarly, I tried to set myself up from a different machine, but again I’m getting the same permission denied error.

I read some other tutorials that suggested to set a password authentication, but that’s not what I want to do. Need ssh login.

Any tips?