Grow Room fancy tile flooring. DWC. Waterproof flooring?

Hes growing in one of his bedrooms and doing DWC. Hes got some fancy tile flooring. Would be pointless to paint with epoxy or anything like that. It looks expensive.

Told him to get pond liner and lay it down nicely to protect the fancy flooring in case of a spillage.

Any other ideas of something to lay down on the fancy flooring to protect it in case of a spillage? or will Pond liner do the job?

The pond liner should give you plenty of protection.
Just spend a little time laying it out and seal it well.

Pond liner will work, but so will a piece of linoleum. You can buy scrap remnants from your local carpet/flooring shop in all manner of sizes. Just tell them you need a piece approximately Length x Width, and that it will be used only for protecting another floor, so you want the cheapest piece of scrap they've got.