As an employer, you are completely entitled to pull up an employee for behaving in a way that is detrimental to the business. Yes something needs to go in the T and C but its a basic general expectation and the employee would have no defence by saying "Its not in my T and C"
I would also suggest that she shouldn't be bitching with clients anyway, especially about other clients, even if FB and social networking didn't exist, its just that nowadays its easier to find out about and prove.
Nip it in the bud!!