There's just not enough equity left to incentivize, create an options pool, hire, raise future rounds, etc. It is a Huge red flag, and the likelihood of success decreases dramatically.
I tried to raise once with 28% of the company being gone from our seed round, and it was a major, major concern. If this company ever needed to raise again, they'd be dead in the water. Therefore, the founder ends up with 33% of nothing instead of, say, 5-10% of something, and it's in his or her own best interest to take a smaller piece.
Now 5%? Maybe 10%? That stings, but at least it leaves the company something to work with.
It's more likely what annovikov said, the one leaving has serious doubts about the strategy but got out-voted.
Think of it as two separate companies: Startup A, which you co-founded, and which ended up dissolving, you seeing nothing for your efforts (other than whatever salary you managed to scrape out); and then, Startup B, which you bought a huge number of shares of for the amazing price of $0/share, and now get to smile as those shares appreciate, like any investor. It just so happens that Startup A and Startup B are the same company at different points in their life, but that shouldn't change your views toward the two situations.
This would be a red flag whether he accepted 33%, 10% or 5%.
The question is, is the flag "more red" if he has a higher percentage.
That is absolutely a red flag. If someone owns 25% of the business, they better be working to grow, improve, or otherwise assist the business on a daily basis. If nothing else, out of self-preservation to protect and grow their investment.
If they're a 1% investor, that's a different story.
If it's 25% and the angel doesn't appear to have angel experience, you can bet that future investors are going to want to interview the angel!
- understand the industry;
- understand the go to market/sales approach; or
- understand the current stage of the company and what it takes to move to the next.
And within each of those, they should have relationships, information, and strategies that accelerate the company in some way. It's not just a "look at my linked in and tell me who you'd like to meet" but a focused "I know A, B, and C who beat the problems that you're dealing with. let me get them right now and see if they can offer advice" and then a while later "Oh, you need X. I know a guy who does that. I'll get back to you."
Investors will likely push for a recapitalization to reset the cap table and remove the outstanding equity all together.
Is it still generally the case that the entirety of the options pool for employees is well under 10% (more like 5%)? If that's the case, I get that it may be limiting for the investors, but hiring & building a team is pretty "cheap" from an options standpoint I believe.
The most fair option is already mentioned - to maintain your 33% that makes all three of the founders equal as of today, and then allocate a new share allotment to dilute you out over a new specified vesting period.
If you want to show future investors good will, ask to maintain a seat on the board.