What did they pour down? Did they do so to unclog? Could a plunger or snake or camera be considered first? Had they already reported any problems to you? Has there been problems before this?

You need to check your policy as to what is covered. Blockages may come under normal wear and tear. Or it may be you need to seek costs from the tenant if they've done the wrong thing. Likewise, you may need to take from their bond etc and you may have an excess. Certainly id be getting a second opinion.