iMac Snow Leopard using Apple Mail via Gmail: Can I get rid of "Sender" field?
When she sends an e-mail from mail, make sure she selects the correct e-mail address in the from drop-down, underneath the subject field.

If her Yahoo e-mail address is not there, she needs to add the outgoing mail server in Mail, Preferences, by clicking on the drop-down menu, and choosing edit smtp server list.