There are three founders, the one leaving the company had the original idea but it has evolved considerably since. We have yet to start development or build the product.
the best answer is that you have an equity agreement in place that spells our the answer, but I'm guess you didn't do that. If he had the original idea and has now bailed on the startup, I would think that you need to appease him so that you don't get sued. If it were me, I'd give him 10-20%.
I would tell him ideas mean nothing and its about the execution!
There is no such thing as "the ideas guy"!
Unless he wants to invest heavily so he has something on the line , if not...cut him loose and get on with it man!
(I doubt he will ever try and sue lol)
With three founders and one creating the idea, I would recommend 20% which leaves you and your co-founder with 40% each since you two will be continuing the idea, investing the money into it, and maintaining its existence. Best of luck to you all.
Thanks for the insight everyone.
Zero - 5%. The germ of an idea is worth less than nothing. Consult an attorney and draft an exit letter for him to sign giving up any and all claim. If it's been a short while, less than 12 months, and he's abandoning, he wouldn't have vested in any stock if you'd put all that in place at the beginning.
If you want to be really nice guys, you could give him what you'd give an advisor. Just be aware that you will have to explain this to any future investors who probably won't like it.