You cannot refuse a "service" animal. You really should speak with an attorney regarding your responsibilities under the ADA and the Fair Housing Act.
If we do not have a signed fee agreement I am not your attorney and this is not legal advice.
I agree with the answer above. A tenant may have the legal right to keep an emotional support animal at her residence notwithstanding a "no pet" clause in the lease. Also, a landlord's refusal to make a reasonable accommodation to a tenant to permit her to keep an emotional support animal can be housing discrimination and expose the landlord to liability. However, each case will depend on the facts specific to each situation. I suggest consulting with an attorney.
I am NOT giving you legal advice. Nothing stated here should be construed to establish any sort of attorney-client relationship. My opinion stated here is limited by the information available to me. You should not rely on my statements in determining your course of action.