The exact details are unclear from the original post, but he definitely isn't giving up 40%. If they've only raised the pre-seed (a reasonable inference given the low valuation), then 10% ownership after 18 months points to two co-founders and a combined investor and option pool dilution of 20%. Anything is possible, of course, but unless the deal terms were very non-standard, this scenario makes the most sense.
You're right that 10% isn't necessarily a huge deal for investors, though. Early-round investor models target a specific ownership stake, and the company has to issue the same number of shares for that no matter what the composition of existing shareholders is.
The challenge with founders leaving is more psychological, like an early engineer who's vested a quarter of their 1% grant realizing that they still have to work hard for three years just to get a tenth of what the guy leaving already has. That's an easy way to suffocate the remaining team's motivation. Potential investors will (and should) look into it, but most of the time it's fine.
