Don't try to do this validation with a Validator.
Cross field validations can't really be done with Validators (not without JSF 2.0 or a lot of painful code. Search this forum for it)
Just write the validation as part of the application invoke stage in your save action or whatever button they push.
Alternatively, perform an ajax call with each check of the checkbox that toggles the required attribute of the textArea and rerenders it.