How to solve this dispute with developer

My developers made some customization for workpress site and at the end here is what they said:

"Changes which was required for your theme improvements are not going to added in ThemeForest directory theme, so yes if you update theme with updated theme , you will loss your customization work. Child theme is not used as we improved/ changed code "

It means I can not update my theme nor will loss the custom work.

With this dispute, who should i contact as a 3rd party to solve this dispute?

Thanks,

There’s nothing you can do at this point. You should have asked the developer to create a child theme before the modifications.

I can offer to separate the modifications and create a child-theme for you. If you’re interested in, you can contact me via the link below: