It is very common in this kind of situation for the owner of a bathroom that is being renovated to take a payment for the component of the renovation that the OC would have to undertake anyway (fixing a leaking membrane) while renovating the whole bathroom.
The OC is entitled to ensure that the tiles are fit for purpose and that the work is done by a licensed operator with valid warranties attached. They might only pay for like-for-like tiles (allowing the owner to upgrade to a more expensive tile at their own expense). But otherwise, this has to be seen as a win-win.