Does a foreigner who is a partner in a Thai company need a visa if he doesn't work there?

As there is no 'normal' visa perhaps should clarify...if you are staying 3 months or less a setv is enough. If 6 months or less a METV would be better and a multi entry non b would be best. The non b can be renewed/issued new each year and then you can get re entry permit to come and go as you please. There likely other ways to squeeze out an extra month here and there with visa exemptions and extensions. Wp not necessary if not working.