Converting Inverter Output from Square Wave to Sin Wave
There is no practical solution in your case. Some big changes must be done in the inverter circuit to make it give sinusoidal output. And it is not an easy and cheap work for an end user to do.

There must have been a warning message on the box of the product, giving information about flaws of the product, which is understandable by any non-technical customer. If there wasn't, you can sue them on for it.