My company let go a developer who is the owner of many scripts. IF I disable him, will it have an impact ?

No, an inactive owner doesn't impact the script. AFAIK it only affects the errors notifications when setup as this:

As @Edgar Valdes alluded to, make sure to update your scripts' error handling so that you're getting notified if/when your scripts blow up. Also, it's a good idea to check that saved searches aren't sending "from" that person's email, as those can fail when a user is inactivated. I'm unclear on whether this is a NetSuite thing or an artifact of their email being deactivated, but either way, it ultimately fails to email the search.