Plagiarism website

Good Morning. We have a problem with this GitHub member mastersubhajit who plagiarizes the site https://www.tessea.com with https://mal.mastersubhajit.com/ . Hosting is on GitHub, what’s your solutions ? Thanks

Copying a website aka making a clone of an existing website is not forbidden as far as I know.
On GitHub you will find plenty of clone repos.
Therefore GitHub wil probably don’t help you with this.

You have to go the legal way if you want to take it down.

and indicate that he designed and developed the site, is it legal?

Possibly not, depending on copyright and trademark law in the relevant jurisdiction(s). But the point is that that is something to talk to a lawyer about, who can advise you on the relevant laws, what is and isn’t legal, and help you file whatever legal documents (e.g. DMCA notice, lawsuit, …) you might choose to use.