A disabled control tells the user:
That pretty much describes your example where simply changing an adjacent option makes input possible. It’s better to use disabling rather than hiding because it helps the user anticipate what is necessary for certain options. Whenever disabling is used, you should make it clear what specifically enables the control. In your example, it’s probably sufficient if you use radio buttons so the text box is right next to the Someone Else option. Worse case, use a text label or tooltip to indicate how to enable the control. For commands (menu items and command buttons), it may even be preferred to leave the control enabled and display an error message explaining what needs to be done first, although that should only be done as a last resort.
Unless there is some simple action the user can do to get access to the control (e.g., by selecting some other controls in the same window), do not use disabling. Rather, hide the controls. For example don’t use disabling for controls users of a certain job or class are never allowed to use. Disabling may confuse some users who will then hunt around the window for something to do to enable them. As far as the users are concerned, actions they don’t have the rights for are not their job (otherwise they’d have access), and so the associated controls should simply not exist in their UI. Documentation or organization procedure manuals may tell users how such actions are accomplished.
Hiding should also be considered as an alternative to disabling when you’re replacing access to one set of controls with access to another set. This is to avoid cluttering your window with excess unavailable controls.
In your particular example there is yet another alternative, assuming you’re using radio buttons: Leave the text box enabled. If users type in the text box, automatically select the Someone Else radio button for them. This saves the user a click. MS Office does this with the Pages radio button and text box in its Print dialogue.
I’ve far more than you ever thought was possible on the subject at Controlling Your Controls.
For similar issues and answers related to disabling and hiding, see also:
Fair to call it a perennial question.