I am not an IL attorney, and you should consult one for a final answer, so my answer should be considered general guidance, not legal advice in your particular case. Also, my answer is not intended to create an attorney-client relationship and is not confidential.
Guardianship usually ends on the date the ward becomes an adult, unless the guardianship relates to an incapacitated person, not a minor. However, since the ward here is related to the employee (you did not day whether the parents are living) and may be a dependent even after reaching adulthood, the new health care reform law, which extends eligibility to 26, may be applicable. Again, you did not say whether you looked at that.
Guardianship would terminate when the girl turns 18, unless she is disabled or otherwise legally incapacitated. If the insurance is based on her being the employee's ward, then she would no longer qualify.
As the other respondent mentioned, however, if she is deemed to be a "dependent" of the employee, and if she remains in school, he/she may be able to continue coverage on that basis. You should check your company's policy, as it may provide your answer, directly.