Copyright issue for DMCA

Our designer was working 2 companies at a same time.He implemented some idea for both companies.
Our company submitted this product first rather than other company and our item is already approve.

The other company called us and told that this designer was working for them and implement some idea for some section for both company.

They told us to change this section but our item is submitted and approved first. Why will we change ?
If need,they will change the design. Because they did not submit till now (they told us).

I asked to them for the proof, they said that file creating is the main proof.
Will it take as poof by themeforest ?
Is it really true to proof the issue?

I don’t understand why you listen to rubbish stories such as the one you are being told.

You may thank them kindly for letting you know that they are in possession of design work that resembles one of your commercial works. They must delete all copies immediately or you will be submitting a DMCA take down if they don’t.

Additionally tell them that it is a crime that is not taken lightly to file false DMCA claims which will only hold them liable for damages.