I've seen it quite often that people refer to someone as a "former founder" or "former co-founder," but that's not really possible since a founder is always a founder. Once you found something, even if you leave the project that you founded, you are still a founder. You can't be a person who was at one time the founder of a project, but is no longer the founder. What's the proper way to refer to this phenomenon?
I would suggest founder emeritus.
The Compact Oxford English Dictionary defines emeritus as an adjective meaning:
(of the former holder of an office, especially a university professor) having retired but allowed to retain their title as an honour
The title for a founder who ceases to work at the company is:
Founding the company is a specific, immutable act that cannot be "undone" after the person leaves the company.
The "past relationship" should be indicated in some other way, such as "Founder and former CEO" or "Co-Founder, who left the company in 1977".
"Founder Emeritus" as suggested in the other answer is ok for a condensed title, although in my opinion it suggests that an official title of "founder emeritus" was awarded to the founder by the company. I do not think that term would be appropriate for use without such an official grant by the company. That's also a title, unlike "founder", that could be revoked.