In most instances like this the co-founder is likely to continue vesting as normal. Whether all parties are happy with this arrangement or not is likely to vary significantly from case to case.
i.e. if one co-founder cannot work as much as the other, should there be a change in compensation?
You can make sure that "your people" (operating from the mindset of corporate feudalism) are taken care of, that if they die prematurely from illness or accident their families are taken care of, and that if they are rendered simply unable to work they and their families are not worse off than in the worst case, without giving someone multi-digit ownership of the company.
It's something to consider when you start drawing up paperwork for your own startup, grisly as that may seem. What happens when one of your founders is struck by a bus a day, a month, a year, five years into the venture? How much of their vested and unvested founder-share goes to their heirs? Does it make a difference if their heirs are their children, their spouse, their siblings, or their favorite charity? Do you want to turn their founder-share from your super-voting shares to a share class with less voting power upon transfer [is that even legal? with vested or unvested shares?]? Does it make a difference if they are killed or merely incapacitated?
It's not a pleasant thing to think about, and it won't matter most of the time, but when it does...
In the first case, you might think "eh, they'll be fine anyway." But the magnitude of the money is not directly related to what is a fair reduction.