It is very important to make sure that you instruct your tenants to inform you of any maintenance issues that arise at the rental unit. When beginning a tenancy, provide your tenant with the Landlord and Tenant Board Brochure on Maintenance and Repairs which can be found at https://tribunalsontario.ca/documents/ltb/Brochures/Maintenance%20and%20Repairs%20(EN).html  

In response to this situation, you can provide your tenant with a letter to remind them that they are to contact you about any maintenance issues before taking the matter into their own hands, and are to provide you with a reasonable amount of time to respond and address the issue(s). Make sure to include your contact information in this letter as well, so that the tenant is aware of how they can reach you. As the landlord, you should be provided with the opportunity to address the situation and provide a solution.

Obtain another quote on the work that was done to see if the bill the tenant sent you is for a reasonable amount.  If it is, you can reimburse the tenant the full amount.  If it is not, you can offer the tenant a different amount.  If the tenant does not find this acceptable, they can file an application against you at the Landlord and Tenant Board to bring the matter in front of an Adjudicator who will make a final decision.