user password not working

I have a multisite network I set up for a client. I’m the superadmin. My client is an Administrator. She has clients who will have sites on the network selling stuff.

I created and set up a new site for one of her clients (this is the first such site on the network).

*My* client was already a user on the network, of course, so I just added her as an “administrator” on the new site. She wanted to log into the new site to do some editing, but says her username/password didn’t work.

Anybody shed some light (or a link to something) on how passwords work in multisite. I thought as an admin, her main log-in info would work on all of the sites on the network — as long as she was added as a user on that particular site … not true?