Changing domain

Hello, I have a question about new domain and traffic

I have a client lawyer that have an old website created from an intranet in 2007. This domain brings 50-100 visitors by month.
He wants a new website and leave the intranet. So he will have a new domain too.

We he lose all his traffic and reboot his SEO ? Or google will remember him maybe with the GMB ?

Thanks for your help !

After you completed the new website (if it contains same content, make sure you hide it from G crawlers), make sure you do the 301 redirects from the old site to the new one, through Google Search Console.
In the ideal scenario, you should make sure you respect the old site structure:
oldsite.com/about/ > newsite.com/about/
oldsite.com/areas-of-expertise/ > newsite/com/-areas-of-expertise/
oldsite.com/blog/ > newsite.com/blog/ (and the structure below to the /blog/)

If your client have oldsite.com/news/ or oldsite.com/legal-corner/, and the new Blog page is newsite.com/blog/, make sure you redirect the old link to the new one.

We he lose all his traffic and reboot his SEO ? Or google will remember him maybe with the GMB ?

You cannot bet on the G's "memory", so make sure you do the right thing.

In this process, you have to wait a few weeks until everything is fine. If the new website is better than the old one (and it should be..), your client may get more potential clients per day. Through "is better", I am talking about speed optimization, improved content with quality images and better text, interlinking, and so on.

As pointed above just do 301 redirect from your domain manager or use htaccess.
If it's WordPress you have plugins that can do that redirect for you.

Ok thanks to you both !