If that actually becomes material, they'll offer to buy shares in the next round. That's the point at which this whole conversation becomes interesting; right now, it's complexity for its own sake.
I know the feeling! I left a company some years back in a complicated way, and my instinct was to drill in as well. It seems like a big deal! It really isn't, though.
Or they'll find a way to dilute the co-founder's shares so they don't have to buy them out.
If that's going to happen, it's going to happen. I've heard as many stories of it happening as I've heard stories of people unhappy with the amount of liquidity they were able to achieve early in the life of a company that later became successful.